Visto che hai solo 7 voci puoi fare in questo modo:
codice:
        Private Sub ListBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged

        Select Case ListBox1.SelectedIndex + 3
            Case 3
                Form3.Show()
            Case 4
                Form4.Show()
            Case 5
                Form5.Show()
            Case 6
                Form6.Show()
            Case 7
                Form7.Show()
            Case 8
                Form8.Show()
            Case 9
                Form9.Show()
        End Select

    End Sub