Developers can get this latest build from the Apple Developer Center, or by going to the Updates section of the Mac App Store on a Mac already running the latest developer beta. There, developers can also find the latest Yosemite Recovery Update. AppleSeed testers can also download the latest build from the Mac App Store.

OS X 10.10.3 new seed

OS X 10.10.3 contains the new Photos for Mac app. The app will let you view, manage, and edit your iCloud Photo Library on your Mac, and will replace Apple's current iPhoto and Aperture applications when it launches to the public at some point this spring.

Source: Apple Developer Center