This is the first ALPHA version of EasyAdmin 3.0. We intend to publish several alphas and betas before the stable release ... but we intend to do it quickly (it won't take months).
EasyAdmin 3.0 is usable right now, but it's lacking some features that were present in EasyAdmin 2.x. If possible, we will re-add some/all of those missing features during the stabilization phase. Otherwise, we'll add them right after the stable release.
EasyAdmin 3 is 100% incompatible with EasyAdmin 2. The new docs will be merged soon, but you can already read them at #2967