Apple today seeded build 13D21 of OS X 10.9.3 to developers, one week after releasing the second OS X 10.9.3 beta, build 13D17 and two weeks after the first 10.9.3 beta. The beta is available through the Software Update mechanism in the Mac App Store as well as through the Mac Dev Center.
Apple is currently asking developers to focus on Graphics Drivers and Audio. As was discovered with the first beta, 10.9.3 adds new support for 4K displays, offering “Retina” resolutions that improve readability along with support for 60Hz output from the Retina MacBook Pro.