Please note, Plesk has supported the version of Mailman provided by OS vendor by default.
The Mailman 3 is shipped by default on Ubuntu 18 and Debian 10.
Please keep voting if this request is still actual for you.
AAMichael Struller commented
on Ubuntu 18.04 the Mailman 2.* will install from plesk.
In Ubuntu we have an Paket mailman v 2.1.26
and an Paket Mailman3.
On command line install-component mailman3 will work?