Delphi - OnMouseWheel

procedure TForm1.FormMouseWheelDown(Sender: TObject; Shift: TShiftState;

  MousePos: TPoint; var Handled: Boolean);

begin

  VertScrollBar.Position := VertScrollBar.Position + 3;

end;

procedure TForm1.FormMouseWheelUp(Sender: TObject; Shift: TShiftState;

  MousePos: TPoint; var Handled: Boolean);

begin

  VertScrollBar.Position := VertScrollBar.Position - 3;

end;