React Navigation 3.0 is out of RC! We couldn't be more excited for this release and the project's maturity over the past year. The best summary you're going to find on changes and improvements is at the source.
We've personally been using the release candidate for a near production app over the past few weeks and found the transition from 2.x ➡ 3.x to be seamless. If you're using Redux though, watch out 🙊.