//На кой ты тут вызываешь MoveToEx?
POINT points[3];
points[0].x = 7;
points[0].y = 7;
points[1].x = PaintBoxButton->Canvas->ClipRect.Width() - 7;
points[1].y = 7;
points[2].x = PaintBoxButton->Canvas->ClipRect.Width() / 2;
points[2].y = PaintBoxButton->Canvas->ClipRect.Height() - 8;
PaintBoxButton->Canvas->Polygon(points,3);