Apple has released the third beta for iOS 8.3 for developers. Registered developers can grab the beta from Apple's Developer Center, or as an over-the-air update to any device with the previous beta installed.
In addition to iOS 8.3 beta 3, Apple has also released the lastest beta of Xcode 6.3 for Mac. Xcode 6.3 comes with an updated version of Apple's Swift programming language.
Source: Apple Developer Center