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 list-owner@example.se --language en -d newtestlist@listdomain.se

More to follow…