Apple today seeded the first test build of OS X 10.9.3 to registered Mac developers, just over a week after releasing OS X 10.9.2. The new beta, which has a build number of 13D12, is available through the Software Update mechanism in the Mac App Store as well as through the Mac Dev Center.
The last Mavericks update, OS X 10.9.2, added FaceTime Audio, the ability to block iMessage users on the Mac, and a fix for a critical SSL vulnerability. 10.9.2 saw a beta testing period of approximately two months, and it is likely 10.9.3 will also see several beta iterations before a public release.
Article Link: Apple Seeds First OS X Mavericks 10.9.3 Beta to Developers