You can use the Click event of each OptionButton, like the following:
Private Sub OptionButton1_Click()
If OptionButton1.Value Then
Range("A15").Value = 1
End If
End Sub
Private Sub OptionButton2_Click()
If OptionButton2.Value Then
Range("A15").Value = 2
End If
End Sub
Private Sub OptionButton3_Click()
If OptionButton3.Value Then
Range("A15").Value = 3
End If
End Sub
Or, in a separate procedure
Select Case True
Case OptionButton1.Value
Range("A15").Value = 1
Case OptionButton2.Value
Range("A15").Value = 2
Case OptionButton3.Value
Range("A15").Value = 3
Case Else
' no option button is selected
End Select