Public Class Form3
    Private _Zielform As Form2
    Public Property Zielform() As Form
        Get
            Return _Zielform
        End Get
        Set(ByVal value As Form)
            _Zielform = value
        End Set
    End Property
    Private Sub Button1_Click(ByVal sender As Object, ByVal e As _
      System.EventArgs) Handles Button1.Click, _
    Button10.Click, Button11.Click, Button12.Click, Button13.Click, _
    Button14.Click, Button2.Click, _
    Button3.Click, Button4.Click, Button5.Click, Button6.Click, Button7.Click, _
    Button8.Click, Button9.Click
        Dim tb As Control
        tb = DirectCast(sender, Button)
        _Zielform.SetText = tb.Text
    End Sub
End Class

Public Class Form1
 
    Public f2 As Form2
 
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
      Handles Me.Load
        Dim f2 As New Form2
        Dim f3 As New Form3
        f2.TopLevel = False
        f2.Dock = DockStyle.Fill
        f2.Parent = SplitContainer1.Panel1
        f2.Show()
        f3.TopLevel = False
        f3.Dock = DockStyle.Fill
        f3.Parent = SplitContainer1.Panel2
        f3.Zielform = f2
        f3.Show()
    End Sub
End Class