Mailman 3 documentation
Mailman 3 is still pretty new, and most of the docs are built with the packages: Links to all the docs
Unlike Mailman 2, Mailman 3 has been divided into a number of parts:
Mailman Suite refers to the collection of related Mailman projects that provide all the pieces of a typical mailing list management setup.
Mailman Core is the part that delivers the emails and handles the mailing list and user data
Postorius is the web interface that allows users (list members, list admins, moderators) to change their user settings and the settings for their lists.
HyperKitty is the archiver which provides a web interface to access GNU Mailman v3 archives as well as ways interact with the lists. It can behave a little like a web forum, for those who prefer not to use email.
Mailman HyperKitty is a plugin for Mailman Core to communicate with HyperKitty.
Django Mailman3 is a set of Django libraries and templates common to Postorius and HyperKitty. It is new with Mailman 3.1.
MailmanClient is the library that provides official Python bindings for the GNU Mailman 3 REST API. This allows you to write your own front end or scripts for interacting with Mailman. (It's also what Postorius uses to interact with Mailman Core.)
A typical install would use all of these pieces together, but they're intended to be modular so you could replace a single piece if you had specific needs.
How do I install Mailman 3 for production?
See here for more details about various options for production installation.
How do I install Mailman 3 for development?
Everything you need to know on how to setup Mailman 3 for development can be found in The Contributing Guide.
How do I upgrade from Mailman 2 to Mailman 3?