模拟就好了，不过要注意顺序....

type
int=longint;

var
   m     : array[0..20,0..20,0..20] of boolean;
   f     : array[0..20,0..20,0..20] of longint;
   a,b,c : longint;

function get(a,b,c:longint):longint;
begin
if m[a,b,c] then exit (f[a,b,c]);
   if (a<=0) or (b<=0) or (c<=0) then
      exit(1);
   if (a>20)or(b>20)or(c>20) then exit(get(20,20,20));
   if (a<b) and (b<c) then
   begin
      f[a,b,c]:=get(a-1,b,c)+get(a-1,b-1,c)+get(a-1,b,c-1)-get(a-1,b-1,c-1);
      m[a,b,c]:=true;
      exit(f[a,b,c]);
   end;
   f[a,b,c]:=get(a-1,b,c)+get(a-1,b-1,c)+get(a-1,b,c-1)-get(a-1,b-1,c-1);
   m[a,b,c]:=true;
   exit(f[a,b,c]);
end;

begin
   readln(a,b,c);
   writeln('w(',a,', ',b,', ',c,') = ', get(a,b,c));
   readln(a,b,c);
   while not((a=-1)and(b=-1)and(c=-1)) do
   begin
      writeln('w(',a,', ',b,', ',c,') = ',get(a,b,c));
      readln(a,b,c);
   end;
end.