Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
 
        If Not e.KeyChar.IsNumber(e.KeyChar) And Not Char.IsControl(e.KeyChar) _
          Then
            e.Handled = True
        Else
             If TextBox1.TextLength > 0 And TextBox1.TextLength Mod 3 = 0 Then
                TextBox1.Text += "." + e.KeyChar
                e.Handled = True
            End If
        End If
    End Sub