Как сохранить три RichTextBox в один RTF файл?

 Dim SFD As New SaveFileDialog
        SFD.Filter = "RichTextFormat (*.rtf)|*.rtf|Текстовые файлы (*.txt)|*.txt"
        If SFD.ShowDialog = DialogResult.OK And SFD.FilterIndex = 1 Then 'сохранить в формате .rtf
            Dim SavedName = SFD.FileName
            Try
                rt1.SaveFile(SavedName, RichTextBoxStreamType.RichText)
                rt2.SaveFile(SavedName, RichTextBoxStreamType.RichText)
                rt3.SaveFile(SavedName, RichTextBoxStreamType.RichText)
            Catch Exc As Exception
                MsgBox(Exc.Message)
            End Try
        ElseIf SFD.FilterIndex = 2 Then 'сохранить в формате .txt
            Dim SavedName = SFD.FileName
            Try
                System.IO.File.WriteAllText(SavedName, rt1.Text, System.Text.Encoding.Default)
                System.IO.File.AppendAllText(SavedName, rt2.Text, System.Text.Encoding.Default)
                System.IO.File.AppendAllText(SavedName, rt3.Text, System.Text.Encoding.Default)
 
            Catch Exc As Exception
                MsgBox(Exc.Message)
            End Try
        End If

rt1.Rtf = rt1.Rtf & rt2.Rtf & rt3.Rtf
rt1.SaveFile(SavedName, RichTextBoxStreamType.RichText)

Dim RTB As New RichTextBox
RTB.Rtf = rt1.Rtf & rt2.Rtf & rt3.Rtf
RTB.SaveFile(SavedName, RichTextBoxStreamType.RichText)

RTB.Dispose

 RTB.AppendText(rt1.Text & vbCrLf & rt2.Text & vbCrLf & rt3.Text)

Private Sub toolstripbutton2_click(sender as object, e as EventArgs) Handles toolsrtipbutton2.click
If SaveFileDialog1.ShowDialog=DialogResult.OK and SaveFileDialog1.FilterIndex =2 Then
FilePath1 = SaveFileDialog1.FileName
Try 
RTB1. SaveFile(FilePath1, RichTextBoxStreamType.RichText)
Catch ex as Exception 
End try
ElseIf SaveFileDialog1.FilterIndex = 1 Then 
Try 
IO.File.WriteAllLines(Filepath1, RTB1.Lines, System.Text.Encoding.GetEncoding(1251))
Catch ex as exception 
End try
End if
End sub

Timer2_Tick(sender As Object,  e as EventArgs) Handles Timer2.Tick
Dim CurLenghtRTB As Integer = RTB1.Text.Lenght 
If IO.File.Exist(FilePath1) Then 
If RTB1.Text.Lenght > 0 then
If CurLenghtRTB > LengthRTB then
SaveToolStripMenuItem.Enabled=True
SavestripButton8.Enable = true
Else 
SaveToolStripMenuItem.Enabled=false 
SavestripButton8.Enable = False
End if 
End if
End if
End sub