You may want to experiment with Left, Right and Mid functions. For the moment, here's a prosaic, late night, brutish approach:
Assume there's a a command button and a text box named TxtTest on your practice form. Assume there's an 8-character string --- 12345678 -- in TxtTest.
Private Sub CmdTest_Click()
Dim TxtTmp As String
If Len(Me.TxtTest) = 8 Then
TxtTmp = "(" & Left(Me.TxtTest, 3) & ")" & " " _
& Mid(Me.TxtTest, 4, 2) & " " _
& Right(Me.TxtTest, 3)
Me.TxtTest = TxtTmp
End If
End Sub
Regards,
Tim