write to selected (active? focused?) textbox

Question

I have 3 buttons (taster1, taster2, taster3), and 3 textboxes (tbA, tbD, tbS. I'd like every button to write something in every textbox successively. First textBox is focused on form_load, and after user fill it I'd like next textBox to be focused.
tbA.Text=tbA.Text+"A";
tbD.Text=tbD.Text+"D";
tbS.Text=tbS.Text+"S";