Chaos is an ASCII ladder

Operation Flashpoint scripting language, 643 624 bytes

f={l=["""","!","#","$","%","&","'","(",")","*","+",",","-",".","/","0","1","2","3","4","5","6","7","8","9",":",";","<","=",">","?","@","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","[","\","]","^","_","`","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","{","|","}","~"];t=_this;w=t select 0;r={l select random 92};v="";s=v;i=2;while{i<w}do{i=i+1;v=v+" "};p={i=1;while{i<t select 1}do{i=i+1;s=s+call r+v+call r+"\n"}};k=0;call p;while{k<t select 2}do{k=k+1;i=0;while{i<w}do{i=i+1;s=s+call r};s=s+"\n";call p};s}

Ridiculously long because there is no way to create the characters from the character codes.

Call with:

hint ([5, 3, 2] call f)

Output:

The ladder is extra chaotic because the font is not monospaced.

Unrolled:

f =
{
    l = ["""","!","#","$","%","&","'","(",")","*","+",",","-",".","/","0","1","2","3","4","5","6","7","8","9",":",";","<","=",">","?","@","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","[","\","]","^","_","`","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","{","|","}","~"];

    t = _this;
    w = t select 0;

    r =
    {
        l select random 92
    };

    v = "";
    s = v;

    i = 2;
    while {i < w} do 
    {
        i = i + 1;
        v = v + " "
    };

    p =
    {
        i = 1;
        while {i < t select 1} do 
        {
            i = i + 1;
            s = s + call r + v + call r + "\n"
        }
    };

    k = 0;
    call p;
    while {k < t select 2} do 
    {
        k = k + 1;

        i = 0;
        while {i < w} do
        {
            i = i + 1;
            s = s + call r
        };
        s = s + "\n";

        call p
    };

    s
}

05AB1E, 29 bytes

Input taken in the order N, H, W

>*GNUžQ¦©.RIÍFð®.R«X²Öè}®.RJ,

Try it online!

Explanation

>*G                              # for N in [1 ... (N+1)*H)-1] do:
   NU                            # store N in variable X
     žQ                          # push a string of printable ascii
       ¦©                        # remove the first (space) and save a copy in register
         .R                      # pick a random character
           IÍF                   # W-2 times do:
              ð                  # push a space
               ®.R               # push a random ascii character
                  «              # concatenate
                   X²Ö           # push X % H == 0
                      è          # index into the string of <space><random_char> with this
                       }         # end inner loop
                        ®.R      # push a random ascii character
                           J,    # join everything to a string and print

C, 95 bytes

f(w,h,n,i){++w;for(i=0;i++<w*~(h*~n);)putchar(i%w?~-i%w%(w-2)*((i/w+1)%h)?32:33+rand()%94:10);}