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