Or for a generic handler:
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged, TextBox2.TextChanged, TextBox3.TextChanged, TextBox4.TextChanged
If DirectCast(sender, TextBox).Text.Length >= 3 Then
DirectCast(sender, TextBox).Parent.SelectNextControl(ActiveControl, True, True, True, True)
End If
End Sub