updated 01:45 pm EDT, Mon July 7, 2014
Precise changes so far undocumented
[Updated with Yosemite changes] Apple has started seeding third previews of OS X Yosemite and iOS 8 to developers. The former is listed as build 14A283o, and can be downloaded via the Mac App Store or the Mac Dev Center. Beta 3 of iOS 8 is identified as build 12A4318c, and available either as an over-the-air update or through the iOS Dev Center. To go with it, Apple is pushing out new betas of Find my iPhone and Find My Friends.
Both releases are arriving three weeks after the last round of betas. On Friday, Apple warned developers that it would do a wipe of CloudKit databases today, something presumably connected to the new betas.
Update: Among other changes, the third Yosemite beta now officially turns on Dark Mode, which was previously available through a terminal tweak. QuickTime sports an updated icon, hinting at possible changes coming to the rarely-revised core video technology. Certain settings previously available in the System Report to let users know if their Mac was compatible with Continuity, AirDrop and Handoff have been removed in DP3, though they will likely return at a later date.
The beta also features larger app icons for Notifications, a new font for the Notification Center and Safari "Favorites" bar, and a few interface tweaks in System Preferences, iCloud Preferences and Mail.