A lot has changed with ModeShape 3, and our connector framework has been replaced with Infinispan and its ability to cache and persist content (see What's New). We don't yet have a migration guide, but do hope to have one very soon. In lieu of that, I'd recommend reading our new documentation (which is still in work) and several interesting threads, including Backup and restore API (which can be used to migrate from 2.x), Binary values in ModeShape 3, and Next generation federation requirements. Look at other (resent) discussion threads, and feel free to ask specific questions if you're unsure how to do something or if the documentation is lacking (or absent).
We'll be releasing 3.0.0.CR2 in the next few days, and hopefully a final release about a week later.