This update includes developer APIs for the new Force Touch trackpad that is a part of the new 13-inch MacBook Pro and the upcoming new version of the MacBook.

The third beta build has the version number 14D98g. Registered Mac developer can download it now via the Software Update feature in the Mac App Store or from the Mac Dev Center. The final public version of OS X 10.10.3 Yosemite later this spring and will also include the new Photos app for Mac along with other improvements and fixes.

Source: MacRumors