I've been testing GXT3 contained within our GXT2 based system, and it's been playing nice.
If I remember correctly, I even tried adding a GXT2 component within the GXT3 container, and it still played nice in terms of working.
This is ignoring the whole resizing thing ofcourse. But I am able to manually listen to resize events and workaround it until everything is transitioned within our system.
This is nice, as we can transition parts of our system. Hopefully start with the scaffolding and outer app, and then upgrade the views etc as required.