Integers, Assemble!

Python 2, 210 200 bytes

Edit: Works now!

Fills from top to bottom, left to right, starting with the largest numbers. Then, transpose and do it again. Then transpose and print. I had to pad with spaces for the transposition to work, since the lines aren't to their full length yet.

I had trouble getting a nested exec working (to do exec'exec"..."*w\n;...'*2. If someone can figure it out, let me know.

n,w,h=input()
s=[""]*h
for x in 1,2:
    exec"for i in range(h):l=len(s[i]+`n`)<=w;s[i]+=`n`*l;n-=l\n"*w;s=[r.replace(" ","")for r in map(lambda x:`x`[2::5],zip(*[r.ljust(w)for r in s]))];w,h=h,w
print s

Try it online - Uses a modified function so it can run multiple test cases more easily (and it couldn't use exec). Uncomment the other version and modify stdin to see it run.

Less golfed:

def f(n,w,h):
    s=[""]*h
    for x in 1,2:
        for j in[0]*w:
            for i in range(h):
                l=len(s[i]+`n`)<=w
                s[i]+=`n`*l
                n-=l
        s=[r.ljust(w)for r in s]
        s=map(lambda x:`x`[2::5],zip(*s))
        s=[r.replace(' ','')for r in s]
        w,h=h,w
    print"\n".join(s)

JavaScript, 284 259 245 241 240 223 209 205 bytes

// Golfed
let f = (N,W,H)=>eval('a=Array(H).fill("");while(N)g:{s=""+N--;d=s[L="length"];for(i in a)if(a[i][L]+d<=W){a[i]+=s;break g}for(p=0;d;++p){l=a[p][L];for(k=p+d;k>p;)l=a[--k][L]-l?W:l;while(l<W&&d)a[p+--d]+=s[d]}}a');

// Ungolfed
(N,W,H) => {
    a = Array(H).fill(""); // Create `H` empty rows.

    while (N) g : {
        s = "" + N--; // Convert the number to a string.
        d = s[L="length"]; // Count the digits in the number.

        // Loop through the rows trying to fit the number in horizontally.
        for (i in a) {
            if (a[i][L] + d <= W) { // If it fits.
                a[i] += s; // Append the number to the row.
                break g; // This is what a goto statement looks like in JavaScript.
            }
        }

        // Loop through the rows trying to fit the number in vertically.
        for (p = 0; d; ++p) {
            l = a[p][L]; // Get the length of the row.

            // Find `d` adjacent rows of the same length.
            for (k = p + d; k > p; ) {
                // If `a[--k][L] == l`, continue.
                // Else set `l` to `W` so the next loop doesn't run.
                l = a[--k][L] - l ? W : l;
            }

            // Put the characters in position.
            while (l < W && d)
                a[p+--d] += s[d];
        }
    }

    return a;
}

let test_data = [[1,1,1],
                 [6,6,1],
                 [6,2,3],
                 [10,1,11],
                 [10,11,1],
                 [11,13,1],
                 [27,9,5],
                 [183,21,21],
                 [184,2,222],
                 [200,41,12],
                 [1003,83,35]];

for (let test of test_data)
    console.log(f(test[0],test[1],test[2]));

Pyth, 35 bytes

juum+WghHl+dQd~tQN<+.TGmkeHeH)_BvzY

Credits to mbomb007. I used his algorithm. Originally I only wanted to help Steven H., but then I really wanted to see a short version.

Takes N on the first line, and W,H on the second line: Try it online: Demonstration

Found a nasty bug in the Pyth implementation of .[ (my own fault, since I implemented it). Gotta fix it tomorrow. This resulted in +3 bytes.

Explanation:

juum+WghHl+dQd~tQN<+.TGmkeHeH)_BvzY
                                  Y   start with the empty list []
                                      I'll perform all operations on this list. 
                                      Sometimes it is called G, sometimes N. 
                                vz    read the second line and evaluate it: [W, H]
                              _B      bifurcate it with reverse: [[W, H], [H, W]]
 u                                    for each pair H in ^:
                    .TG                  transpose G
                   +   mkeH              append H[1] empty strings
                  <        eH            use only the first H[1] strings
                                         lets call this result N
  u                          )           modify N, until it doesn't change anymore:
   m                        N               map each d in N to:
     WghHl+dQ                                  if H[0] >= len(d+Q):
    +        d  Q                                 d + Q
              ~t                                  and decrement Q by 1
             d                                 else:
                                                  d
j                                     at the end print every row on a separate line