Mailman 3 is a fully rewritten code base. The developers believe it has sufficient functionality to provide full mailing list services. It should be ready for production use by experienced system developers, but it may not be easy to install or run by novices.
It should also be possible to migrate Mailman 2.1 mailing lists to Mailman 3. Caution, backups, and testing is recommended.
We expect it to be possible to run Mailman 3 and Mailman 2.1 together on the same systems, but you may need to be quite experienced with configuring your mail server and web infrastructure.
Mailman 3 may have bugs.
Mailman 3 is not yet feature complete with Mailman 2.1.
The documentation here describes the Mailman Core in great detail. Postorius, Hyperkitty, and mailman.client are described and developed elsewhere.
More release notes are maintained on the Mailman wiki.