Se hai 3 textbox, nominale tutte Text1, e imposta la prop Index, creando una matrice di controlli.

codice:
Text1_Keydown(Index as integer,KeyCode As Integer, Shift As Integer)
if keycode=vbkeydown then
if index > numerotextbox -1 then
     text1(0).setfocus
else
     text1(index + 1).setfocus
end if
elseif keycode=vbkeyup then
if index =0 then
     text1(numerotextbox-1).setfocus
else
     text1(index - 1).setfocus
end if
end if