The short answer is yes. You can take a look at this post. As far as I understand, any java app can work on app engine. Spring is no exception, and as far as I understand, spring is not supported out of the box by app engine, so you are free to use whatever version you want to use.
It seems other people are answering before I can...and that's fine with me, especially since I don't have an authoritative answer on Spring 3 and GAE (I'm kinda new to GAE...just setup my account 2 days ago and still haven't done anything with it.)
But, from what I understand, Spring 3 should work just fine on Google App Engine, so long as you don't do anything that creeps beyond GAE's special constraints (no file system access, Big Table, etc).