Print ASCII Voxels

Lua (1442 bytes)

Bonus animations! :)
If you have any cool voxel art in the same format as the examples, link it in the comments and I'll make an animation out of it
7x7x7
Bonus animation
12x12x12
Bonus 2
This is my first code golf, so its quite messy and I plan on improving it or porting it to a different language.
Here is what i have, right now at just under 2.5kB barely-golfed (just removed indentation whitespace at this point, I will continue more later)

Here is the now ~1.4kB golfed and minified version (note the "a" table on the first line is the placeholder for the voxel matrix):

local a={}
local b,c=table,string;local d,e,f,g,h,i=b.concat,#a,c.gsub,c.gmatch,ipairs,b.insert;local function j(k,l,m)return a[k]and a[k][l]and a[k][l][m]==1 end;local n={}for o=1,e*5+1 do n[o]={}for p=1,e*7+1 do n[o][p]=" "end end;local q=[[
__6hhhh7
_g    ij
1aaaa2 j
b    d 5
b    de_
3cccc4__
]]local function r(k,l,m)local s=q;if j(k,l,m)then local t,u,v,w,x,y,z,A=j(k-1,l,m),j(k+1,l,m),j(k,l,m-1),j(k,l,m+1),j(k,l-1,m),j(k+1,l+1,m),j(k+1,l,m+1)," "if t then s=f(s,"[ai]"," ")end;if u and not y then A=A.."c"end;if u and not z then A=A.."e"end;if v then A=A.."bg"end;if w then A=A.."di"end;if x then if not j(k,l-1,m+1)then A=A.."j"end;A=A.."h"end;if t then if w and j(k-1,l,m+1)then A=A.."2"end;if v and j(k-1,l,m-1)then A=A.."1"end end;if u then if w and j(k+1,l,m+1)then A=A.."4"end;if v and j(k+1,l,m-1)then A=A.."3"end end;if x then if w and j(k,l-1,m+1)then A=A.."7"end;if v and j(k,l-1,m-1)then A=A.."6"end;if u and j(k+1,l-1,m)then A=A.."5"end end;s=f(f(f(f(f(s,"["..A.."]"," "),"[ach]","-"),"[bdj]","|"),"[gie]","/"),"[1234567]","+")else s=nil end;return s end;local B,C;local D=e*2-1;local E=1;for k=e,1,-1 do for l=1,e do for m=1,e do B=(l-1)*-2+(m-1)*5+D;C=(l-1)*2+(k-1)*3+E;local s=r(k,l,m)if s then local F={}for G in s:gmatch("[^\n]+")do local H={}for I in G:gmatch(".")do i(H,I)end;i(F,H)end;for J,K in h(F)do for L,I in h(K)do if I~="_"then n[C+J-1][B+L-1]=I end end end end end end end;for o,a in h(n)do print(d(a))end

Edit: Here is the original (over 3kB) ungolfed version, including my edits for making the animation (if you are running it yourself and want the animation, change the false near the bottom of the code to true.

local v = {}
local depth = #v;

function voxel(y,z,x)
  return (v[y] and v[y][z] and v[y][z][x]==1)
end

local canvas = {}
print("Constructing canvas of depth",depth)
for i=1,depth*5+1 do
  canvas[i] = {}
  for j=1,depth*7+1 do
    canvas[i][j] = " "
  end
end

local voxelProto = [[
__6hhhh7
_g    ij
1aaaa2 j
b    d 5
b    de_
3cccc4__
]]

function renderVoxel(y,z,x)
  local vox = voxelProto
  if (voxel(y,z,x)) then
    local up = voxel(y-1,z,x)
    local down = voxel(y+1,z,x)
    local left = voxel(y,z,x-1)
    local right = voxel(y,z,x+1)
    local back = voxel(y,z-1,x)
    local downFront = voxel(y+1,z+1,x)
    local downRight = voxel(y+1,z,x+1)

    if (up) then
      vox = vox:gsub("[ai]"," ")
    end
    if (down and not downFront) then
      vox = vox:gsub("[c]"," ")
    end
    if (down and not downRight) then
      vox = vox:gsub("[e]"," ")
    end
    if (left) then
      vox = vox:gsub("[bg]"," ")
    end
    if (right) then
      vox = vox:gsub("[di]"," ")
    end
    if (back and not voxel(y,z-1,x+1)) then
      vox = vox:gsub("[j]"," ")
    end
    if (back or up) then
      vox = vox:gsub("[h]"," ")
    end
    if (up and right and voxel(y-1,z,x+1)) then
      vox = vox:gsub("[2]"," ")
    end
    if (up and left and voxel(y-1,z,x-1)) then
      vox = vox:gsub("[1]"," ")
    end
    if (down and right and voxel(y+1,z,x+1)) then
      vox = vox:gsub("[4]"," ")
    end
    if (down and left and voxel(y+1,z,x-1)) then
      vox = vox:gsub("[3]"," ")
    end
    if (back and right and voxel(y,z-1,x+1)) then
      vox = vox:gsub("[7]"," ")
    end
    if (back and left and voxel(y,z-1,x-1)) then
      vox = vox:gsub("[6]"," ")
    end
    if (back and down and voxel(y+1,z-1,x)) then
      vox = vox:gsub("[5]"," ")
    end

    vox = vox:gsub("[ach]","-")
    vox = vox:gsub("[bdj]","|")
    vox = vox:gsub("[gie]","/")
    vox = vox:gsub("[1234567]","+")
  else
    vox = nil
  end
  return vox
end
local xpos,ypos
local minx = depth*2-1
local miny = 1;

local pic = {}
function drawCanvas()
  for k,v in pairs(canvas) do
    pic[k] = table.concat(v)
  end
  return table.concat(pic,"\n")
end

local timeline = {}
print("Compositing voxels")
for y=depth,1,-1 do
  for z=1,depth do
    for x = 1,depth do
      xpos = (z-1)*-2 + (x-1)*5 + minx
      ypos = (z-1)*2 + (y-1)*3 + miny
      local vox = renderVoxel(y,z,x)
      if (vox) then
        local vt = {}
        for line in vox:gmatch("[^\n]+") do
          local vtl = {}
          for c in line:gmatch(".") do
            table.insert(vtl,c)
          end
         table.insert(vt,vtl)
        end
        for ly,chars in ipairs(vt) do
          for lx,c in ipairs(chars) do
            if (c ~= "_") then
              canvas[ypos+ly-1][xpos+lx-1] = c
            end
          end
        end
        table.insert(timeline,drawCanvas())
      end
    end
  end
end

if (false) then -- change to true if you want to see the animation!
  for i=1,#timeline do
    local t = os.clock() + 0.05
    io.write(timeline[i],'\n\n')
    io.flush()
    while (t > os.clock()) do end
  end
end         
print(timeline[#timeline])

Here is a sample of code that will populate the voxel matrix from a string for a 3x3x3 voxel matrix. (It will take any string in a similar format, but make sure it is a cube or things will probably break.)
To use this, insert this chunk right after the first line local v = {}

local vs = [[
100
000
000

110
100
000

111
110
101
]]
for layer in vs:gmatch("[^a]+") do
 local a = {}
 for row in layer:gmatch("[^\n]+") do
 local b = {}
 for _vox in row:gmatch("[01]") do
 table.insert(b,(_vox=="1") and 1 or 0)
 end
 table.insert(a,b)
 end
 table.insert(v,a)
end

Here is the output from the given 12x12x12 voxel pattern: (and yes it does look better on a normal console/text viewing apparatus, there is a bit too much vertical spacing here)

                                                                          +----+----+
                                                                         /         /|
                                                                        +----+----+ |
                                                                        |         | +
                                                            +----+      |         |/
                                                           /    /|      +    +----+
                                                          +----+ |      |    | +----+
                                                          |    | +      |    |/    /|
                                                          |    | |      +    +----+ |
                                                          +    + |      |         | +
              +----+----+                         +----+--|    | +      |         |/
             /         /|                        /        |    | |      +    +----+
            +----+----+ |                       +----+----+    + |      |    | +----+
            |         | +                       |              | +      |    |/    /|
            |         |/       +----+----+----+ |              | |      +    +----+ |
            +    +----+       /              /| +    +----+    + |      |         | +
            |    | +         +----+----+----+ | |    | +--|    | +      |         |/
            |    | |         |              | + |    |/   |    | |      +----+----+
            +    + |         |              | | +    +----+    + |            
            |    | +         +    +----+    + | |              | +            
            |    | |         |    | +--|    | + |              |/             
            +    + |         |    |/   |    | | +----+----+----+              
            |    | +----+    +    +----+    + |                               
            |    |/    /|    |              | +                               
            +    +----+ |    |              |/                                
            |         | +    +----+----+----+                                 
            |         |/                                                      
            +----+----+                                       +----+----+     
                                                             /         /|     
                                                  +----+    +----+----+ |     
                                                 /    /|    |         | +     
                                                +----+ |    |         |/      
                                                |    | +    +    +----+       
      +----+----+----+  +----+----+----+----+---|    | |---+|    | +----+-+----+----+
     /              /| /                        +    + |    |    |/    /|          /|
    +----+----+----+ |+                         |    | +    +    +----+ |         + |
    |              | +                          |    | |    |         | +        /  +
    |              | |      +----+----+----+    +    + |    |         |/        +  /
    +    +----+    + |     /              /|    |    | +    +    +----+        /  +
    |    | +--|    | +    +----+----+----+ |    |    | |    |    | +          +  /
    |    |/   |    | |    |              | +    +    + |    |    | |         /  +
    +    +----+    + |    |              | |    |    | +    +    + |        +  /
    |              | +    +    +----+    + |    |    | |    |    | +       /  +
    |              | |    |    | +--|    | +    +    + |    |    |/       +  /
    +----+----+    + |    |    |/   |    | |    |    | +    +----+       /  + 
      +----+--|    | +    +    +----+    + |    |    |/                 +  /  
     /        |    | |    |              | +    +----+                 /  +   
    +----+----+    + |    |              |/                           +  /    
    |              | +    +----+----+----+                           /  +     
    |              |/                                               +  /      
    +----+----+----+                                               /  +       
      +                                                           +  /        
     /                                                           /  +         
    +                                                           +  /          
   /                                                           /  +           
  +                                                           +  /            
 /                                                           /  +             
+----+----+----+----+----+----+----+----+----+----+----+----+  /              
|                                                           | +               
|                                                           |/                
+----+----+----+----+----+----+----+----+----+----+----+----+   

Here is the output from the 7x7x7 example here

              +----+----+----+    +----+----+----+
             /              /|   /              /|
            +    +----+    + |  +    +----+    + |
           /    /|   /    /  + /    /|   /    /  +
          +    + |  +    +  / +    + |  +    +  / 
         /    /  +-/    /  + /    /  +-/    /  +  
        +----+  /-+----+  /-+----+  /-+----+  /--+
        |    | +  |    | +  |    | +  |    | +  /|
      +----+ | |+----+ | |+----+ | |+----+ | | + |
     /    /| + /    /| + /    /| + /    /| + |/  +
    +    + | |+    + | |+    + | |+    + | | +   |
   /    /  + /    /  + /    /  + /    /  + |   + |
  +    +----+    +   |+    +----+    +   | +  /| +
 /              /  + /              /  + | | + | |
+----+----+----+  /|+----+----+----+  /| + |/--+ |
|              | + ||              | + | |-+  /  +
|              |/--+|              |/--+ |   +  / 
+----+----+----+  / +----+----+----+  /  +  /  +  
  +    +  / +    +----+    +  /-+    +  /--+  /--+
 /    /  + /              /  + /    /  +   | +  /|
+----+  / +----+----+----+  /-+----+  /--+ |/  + |
|    | +  |              | +  |    | +  /|-+  /  +
|    | |-+|              |/  +|    | | + |   +  / 
+    + |  +----+----+----+  /|+    + |/  +  /  +  
|    | +----+----+ | |+    + ||    | +  /--+  /   
|    |/         /| + /    /  +|    |   +   | +    
+    +----+----+ | |+----+  /-+    +  /--+ |/     
|              | + ||    | +  |    | +  /|-+      
|              | | +|    | | +|    |/  + |        
+----+----+    + | |+    + |/|+----+  /  +        
  +    +--|    | + ||    | + |  +    +  /         
 /        |    | |-+|    |   +-/    /  +          
+----+----+    + |  +    +  / +----+  /           
|              | +  |    | +  |    | +            
|              |/   |    |/   |    |/             
+----+----+----+    +----+    +----+              

Python ( 444 361 354 bytes)

Edit: I found another bug that would omit a corner cross in very special cases. A straight forward fix added 50 bytes to the code so I tried to golf it down a bit further. Now the bug is fixed and it's even 83 bytes shorter. It's becoming very hacky. I also wanted to get rid of the fivefold for loop but could not yet find a solution. Any ideas?

Edit 2: With a very long import I can save another 7 characters.

Neither very short nor very elegant, but then again it is a complex problem:

#input:
u=[[[1,1,1],[1,0,1],[1,1,1]],
   [[1,0,1],[0,0,0],[1,0,1]],
   [[1,1,1],[1,0,1],[1,1,1]]]

#actual code that counts:
r=range
n=len(u)
g=r(n)
a=([' ']*9*n+['\n'])*6*n
t='/|-+\n '
d=dict(zip(t+'%!=*',2*t))
for y,z,x,i,j in __import__('itertools').product(g,g,g[::-1],r(6),r(8)):
 if abs(i+j-6)<5*u[x][y][z]:a[(9*n+1)*(i+3*x+2*y)+j+5*z-2*y+2*n]+='./    %|+====* ||    ! *|    !/.*----+'[8*i+j-8]
o=''.join((d[x[-1]],' ')[x[-2:]in('%/','!|','=-')or x[-4:]=='*++*']for x in a)

#output:
print o

It first draws all the individual voxels with all lines on top of each other. Then it outputs only the topmost characters and gets rid of those lines and crosses on planar planes that should not be drawn according to the specs.

I guess it's possible to golf this down quite some more, but 444 is such a nice number. :)

Example 3x3x3 and 7x7x7 output (with some newlines removed):

        +----+----+----+   
       /              /|   
      +    +----+    + |   
     /    /|   /    /  +   
    +    +----+    +   |   
   /              /  + |   
  +----+----+----+  /| +   
  |              | + | |   
  |              | |-+ |   
  +    +----+    + |/  +   
  |    | +--|    | +  /    
  |    |/   |    |   +     
  +    +----+    +  /      
  |              | +       
  |              |/        
  +----+----+----+         

                +----+----+----+    +----+----+----+           
               /              /|   /              /|           
              +    +----+    + |  +    +----+    + |           
             /    /|   /    /  + /    /|   /    /  +           
            +    + |  +    +  / +    + |  +    +  /            
           /    /  +-/    /  + /    /  +-/    /  +             
          +----+  /-+----+  /-+----+  /-+----+  /--+           
          |    | +  |    | +  |    | +  |    | +  /|           
        +----+ | |+----+ | |+----+ | |+----+ | | + |           
       /    /| + /    /| + /    /| + /    /| + |/  +           
      +    + | |+    + | |+    + | |+    + | | +   |           
     /    /  + /    /  + /    /  + /    /  + |   + |           
    +    +----+    +   |+    +----+    +   | +  /| +           
   /              /  + /              /  + | | + | |           
  +----+----+----+  /|+----+----+----+  /| + |/--+ |           
  |              | + ||              | + | |-+  /  +           
  |              |/--+|              |/--+ |   +  /            
  +----+----+----+  / +----+----+----+  /  +  /  +             
    +    +  / +    +--- +    +  /-+    +  /--+  /--+           
   /    /  + /              /  + /    /  +   | +  /|           
  +----+  / +----+----+----+  /-+----+  /--+ |/  + |           
  |    | +  |              | +  |    | +  /|-+  /  +           
  |    | |-+|              |/  +|    | | + |   +  /            
  +    + |  +----+----+----+  /|+    + |/  +  /  +             
  |    | +----+----+ | |+    + ||    | +  /--+  /              
  |    |/         /| + /    /  +|    |   +   | +               
  +    +----+----+ | |+----+  /-+    +  /--+ |/                
  |              | + ||    | +  |    | +  /|-+                 
  |              | | +|    | | +|    |/  + |                   
  +----+----+    + | |+    + |/|+----+  /  +                   
    +    +--|    | + ||    | + |  +    +  /                    
   /        |    | |-+|    |   +-/    /  +                     
  +----+----+    + |  +    +  / +----+  /                      
  |              | +  |    | +  |    | +                       
  |              |/   |    |/   |    |/                        
  +----+----+----+    +----+    +----+