3.0 is pretty much a re-write of everything that was poorly coded in Pocketmine. It will be a ton more efficient and easy to use, but unfortunately, it will render most old plugins unusable.
API 2.1.0 has mostly changed metadata stuff for entities. Data flags have changes, and it is no longer recommended to use those in your plugin where possible. API 3.0.0 is nowhere near finished. Currently there have been a bunch of changes to chunks, but any plugins from 2.1.0 that do not have to do with chunks should mostly work CURRENTLY. There are loads of changes yet to come however, so most likely there's way more going to change.
That's a complete changelog somewhere. Your mission... If you choose to accept it, is to find it without help. I'm not being facetious, I'm still looking :-/