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Uplně jednoduše pomocí klasického rozeskoku Select Case...

Public Class Form1
  Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
    Select Case ComboBox1.SelectedIndex
      Case 1
        TextBox1.ReadOnly = True
        TextBox2.ReadOnly = False
        TextBox3.ReadOnly = False
      Case 2
        TextBox1.ReadOnly = False
        TextBox2.ReadOnly = True
        TextBox3.ReadOnly = False
      Case 3
        TextBox1.ReadOnly = False
        TextBox2.ReadOnly = False
        TextBox3.ReadOnly = True
    End Select
  End Sub
End Class