As far as I know, there is no MVC-pattern in GXT 3.0. :-)
You can choose any mvp framework you like. Use native GWT MVP with Activites & Places, or choose one of the MVP frameworks like GWT-Platform or MVP4G. I prefer mvp4g. This framework works really nice with GXT 3.0.
In the mvp4g examples, there is one example, that uses GXT (in version 2.x).