Maybe I did not understand my task right?!
Here is my program:

program p1164;
var
   n,m,p : integer;
   a : array['A'..'Z'] of integer;
   i : integer;
   j : integer;
   ch : char;
begin
  fillchar(a,sizeof(a),0);
  readln(n,m,p);
  for i:=1 to n do
  begin
    for j:=1 to m-1 do
    begin
      read(ch);
      inc(a[ch]);
    end;
    readln(ch);
    inc(a[ch]);
  end;
  for i:=1 to p do
  begin
    for j:=1 to m-1 do
    begin
      read(ch);
      dec(a[ch]);
    end;
    readln(ch);
    dec(a[ch]);
  end;

 for ch:='A' to 'Z' do
 begin
   for i:=1 to a[ch] do Write(ch);
 end;
 writeln;
end.

> Maybe I did not understand my task right?!
> Here is my program:
>
> program p1164;
> var
>    n,m,p : integer;
>    a : array['A'..'Z'] of integer;
>    i : integer;
>    j : integer;
>    ch : char;
> begin
>   fillchar(a,sizeof(a),0);
>   readln(n,m,p);
>   for i:=1 to n do
>   begin
>     for j:=1 to m-1 do
>     begin
>       read(ch);
>       inc(a[ch]);
>     end;
>     readln(ch);
>     inc(a[ch]);
>   end;
>   for i:=1 to p do
>   begin
>     for j:=1 to m-1 do
>     begin
>       read(ch);
>       dec(a[ch]);
>     end;
>     readln(ch);
>     dec(a[ch]);
>   end;
>
>  for ch:='A' to 'Z' do
>  begin
>    for i:=1 to a[ch] do Write(ch);
>  end;
>  writeln;
> end.