The Mailman 3 Release note say "While it is possible to upgrade existing Mailman 2.1 lists to run on Mailman 3, we are not yet officially sanctioning it. The plan is to officially support upgrades with Mailman 3.1, but for now, feel free to experiment with upgrading and let us know how it goes. It should be possible to run Mailman 2.1 on the same site as Mailman 3."
As an active mailinglist user I want to be reassured nothing gets lost or broken in an upgrade from Mailman 2.1 to Mailman 3.0.