Apple has released both the third public beta, along with the eighth developer preview for OS X Yosemite. If you're already on the public beta, open the Mac App Store and check for updates. The public beta began in July, with the second beta arriving late last month.
Developers can download Developer Preview 8 from the Apple Developer Center. OS X Yosemite brings a flatter look more in line with iOS, along with features like Continuity, which lets your Mac, iPhone, and iPad work more closely together. It is expected to be available in full public release later this year.