The beta is available through the Software Update mechanism in the Mac App Store as well as through the Mac Dev Center.
Apple continues to ask developers to focus on Graphics Drivers, Audio, Mail, Contacts and Calendar sync over USB in iTunes, and Safari. 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.
OS X 10.9.3 is likely nearing completion, as AppleCare employees were today seeded 10.9.3 build 13D62, one build later than what developers are receiving. A public launch could come as soon as this week.