Like previous versions of the Developer Preview, stock software builds are available to flash on the Nexus 5, Nexus 6, Nexus 9, And Nexus Player. You can download the TGZ files for a manual flash from Google's Android Developer page right now. Over-the-air updates should be coming any time now, and we'll be sure to let you know when they do - if you want to flash over your current preview without losing any data, it's best to wait. According to this blog post, the changes are fairly minimal, limited to a simple adjustment to the permissions UI and the Fingerprint API (which shouldn't affect any of the current preview hardware).
As is our wont, Android Police will be poring over any other possible changes in the new phone, tablet, and Android TV builds over the next few days. Stay tuned to see what we find.
Google has posted new versions of the system images for Android 6.0 Preview 3 - supposedly only the system.img has been modified, and no apps need to be optimized on boot, so this is likely just a small system-level tweak fixing some bug or issue Google spotted shortly after releasing the first images. The new build number is MPA44I (the old one is MPA44G), and you can get the new images for said devices at the links below: