The following code will test to see if a number is divisible by three:

{$R *.dfm}

var

  iNumber: Integer;

  bTest:Boolean;

function DivideByThree(iNumber:Integer):Boolean ;

var

  sNumber:string;

  i,iSum: Byte;

begin

  DivideByThree:=False;

  iSum:=0;

  sNumber    := IntToStr(iNumber);

  for i := 1 to Length(sNumber) do

     iSum := iSum + StrToInt(sNumber[i]);

   if (iSum mod 3 = 0) then

      DivideByThree := True;

end;  // end DivideByThree

procedure TForm1.btnTestClick(Sender: TObject);

begin

   iNumber := StrToInt(edtNumber.Text);

   if DivideByThree(iNumber) then

       ShowMessage('Divisible Byte three')

   else

       ShowMessage('Not Divisible Byte three')

end;

end.