The new beta, build 14D98g, is available for registered developers through the Software Update mechanism in the Mac App Store and through the Mac Dev Center. Apple has also released OS X Server 4.1 for developers.
According to the release notes, today's 10.10.3 beta includes developer APIs for the new Force Touch trackpad in the 13-inch Retina MacBook Pro and the upcoming Retina MacBook, which will allow developers to create apps that take advantage of Force Touch.
There's also a new feature that adds additional data sources when using "Look Up." When right-clicking on a highlighted word, Look Up will now search iTunes, App Store, movie showtimes, nearby locations, and more, in addition to the dictionary. Sources included in Look Up can be customized in Preferences. Currently, the feature is buggy on some machines, causing the keyboard to stop working.
Along with the new Photos for OS X app, the first two OS X 10.10.3 betas introduced a redesigned emoji picker that consolidates emoji into a single page with clear labels, new diversified emoji and emoji skin tone modifiers, new flag emoji and updated emoji for the iPhone, iMac, and Apple Watch, and support for Google 2-step verification when setting up accounts in System Preferences.
Article Link: Apple Seeds Third OS X 10.10.3 Yosemite Beta to Developers