请问一道简单的VB题……

这样就可以了。前提是在你的Form上有Text1~Text4、Command1、Command2这六个控件。

Private Sub Command1_Click()
Dim a As Single, b As Single, c As Single, d As Single

a = val(Text1.Text )
b = val(Text2.Text )
c = val(Text3.Text )

d = (a + b + c) / 3

Text4.Text = d
End Sub

Private Sub Command2_Click()
End
End Sub