InfoPath Dev

I have a repeating table with 3 columns.
Ex: Field1,Field2,and Field3

If user changes the value in the Field1 I want to update the Field3 value as Today date.

In the OnAfterChange event I used

    e.Source.selectSingleNode("Field3").text = DateTime.Today.ToString();

 But it getting me a null reference.

Can anybody suggest a better idea to achieve the above task. 

Thanks

I'm guessing it would be something more like "../my:Field3" and not just "Field3", but why not just use an ordinary rule to do this?