iOS 5.1 beta 3 has been released to developers today, the latest build is 9B5141a and is available as a fairly small over-the-air delta update to those currently running iOS 5.1 beta 2. As past iOS 5.1 betas have shown, compatibility is with iPad 2, iPad, iPhone 4S, iPhone 4, iPhone 3GS, and iPod touch 3rd and 4th generations. Registered iOS developers should be able to get the latest build through the Apple Dev Center as well.