Well, I finally figured out my problem. Since the "/db/data/" portion of the URL, I'd assumed that was how you pointed at the database. Now it's become evident that this is just the route for talking to the *active* database, and is the same no matter what you've named your database.
So far it looks like it can talk to 3.0 just fine, though I'd still enjoy hearing confirmation that 3.x should function without too many surprises.