Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   Dim n, m, k As Long
   n = Val(TextBox1.Text)
   m = Val(TextBox2.Text)
   k = Val(TextBox3.Text)
   If n * m * k = 0 Then Exit Sub
   Label1.Text = ComDiv(ComDiv(n, m), k)
End Sub    
    Private Function ComDiv(ByVal n&, ByVal m&) As Long
       Dim c, r As Integer
       If m > n Then c = m : m = n : n = c
       Do
          r = n Mod m
          n = m : m = r
       Loop While r <> 0
       Return n
    End Function
