Create form with three text boxes(text1,text2,text3), one CommandButton(cmdAdd) and one label(label1). Double click on the button and add the following code.
Private Sub cmdAdd_Click()
Dim x1, x2, x3, sum As Single
x1 = Val(Text1.Text)
x2 = Val(Text2.Text)
x3 = Val(Text3.Text)
sum = x1 + x2...