How to UDV linked with multiple fields

Hello!

I want to update Quantity field in some documents automatically adding values of 3 user fields:

Quantity=U_FIELD1+U_FIEDL2+U_FIELD3

I have created a User Defined Value in Quantity field linked with U_FIELD1. It runs Ok, but when I change U_FIELD2 or U_FIELD3, Quantity field is not updated (I have to press Shift+F2 and value is refreshed then). How can refresh automatically without press Shift+F2 when U_FIELD2 or U_FIELD3 are changed?

Thanks a lot.

Regards.