hsr//////////////////////////////////////////////
아래는 재귀호출과 ord,chr를 사용했습니다.
다른방법도 있을건데요...
procedure TForm1.permutation(OrgStr : string; pos : integer);
begin
    if (pos < 1)or(pos > Length(OrgStr)) then exit;
    if (Ord(OrgStr[pos]) <= 90) then begin
       ListBox1.Items.Add(OrgStr);

       if (pos < Length(OrgStr)) then begin
          OrgStr[pos+1]   := 'B';
          permutation(OrgStr,pos+1);
       end
       else begin
          OrgStr[pos] := Chr(Ord(OrgStr[pos])+1);
          permutation(OrgStr,pos);
       end;
    end
    else begin
       OrgStr[pos-1] := Chr(Ord(OrgStr[pos-1])+1);
       OrgStr[pos]   := 'A';
       permutation(OrgStr,pos-1);
    end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
    str : string;
begin
    ListBox1.Clear;
    str := 'AAA';
    permutation(str, Length(str));
end;