' Code von Form1
Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim f As New Form2
        f.MdiParent = Me
        f.Show()
    End Sub
End Class
 
'Code von Form2
Public Class Form2
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim f As New Form3
        f.MdiParent = Form1
        f.Show()
    End Sub
End Class