The most complete documentation seems to be found here.
I had issues creating a new mailing list and it seems that mailman 3 needs to register domains before it will accept a new list to a particular domain. I had the error:
mailman: error: Illegal list name: newtestlist
This was fixed by automatically registering the domain using the -d option:
mailman -C var/etc/mailman.cfg create -o email@example.com --language en -d firstname.lastname@example.org
More to follow…