Hey guys,
We all know this has been annoying as H-E double toothpicks with 7... But I found a quick way that works for me, at least, every time.
First, force-close RR3. Then, go to settings and sign out of GameCenter. Then, before you reopen RR3, go into airplane mode.
Open RR3 - you should be back in as "Guest" every time.
J.