The third Developer Preview of Android O has been released by Google and for those who have registered devices in the Android Beta Program, you should start seeing the update hit your devices. DP3 arrived earlier than expected as Google had slated this for late June. The early June release signals that the company is overall pretty pleased with where it stands in its development and also gives developers more time to prepare their apps.
A key element of the DP3 release is the finalized APIs for developers. That means they can get started on updating or adding functionality to their apps based on these APIs, knowing they are not going to change between now and the final release. That means you can expect over the next few months to see apps start noting updates in the Play Store with support for Android O prior to the release.
A forth and presumed final Developer Preview is expected in the mid-July timeframe with the final version to be released Q3 of this year. That means it should be somewhere between July and September with it likely being towards the end of that timeframe.
If you have a Nexus 6P, Nexus 5X, Nexus Player, Pixel, Pixel XL, or Pixel C you can register your device through the Android Beta Program to try out DP3. As always, remember that this is still beta so you could run into various bugs and issues. I highly recommend not running it on your daily driver so you don’t run into unexpected challenges. Also, because it is preview code, Android Pay will not work (or at best inconsistently).