How to use the galera-3 package correctly in Ubuntu 18 LTS?

apt-get install galera-3 mysql-server is successful and creates /usr/lib/galera/libgalera_smm.so, but show status like 'wsrep%' returns nothing.

When I put wsrep_provider=/usr/lib/galera/libgalera_smm.so into my mysql configuration, service start mysql fails with unknown variable 'wsrep_provider=/usr/lib/galera/libgalera_smm.so'.

When I used Galera several years before in Ubuntu 14 LTS, I had to install a galera enabled mysql binary from an external source, but with the availability of the galera-3 package in the Ubuntu 18 repositories I expected that to have changed. Am I wrong ? Or how can I use the galera-3 package correctly ?

Edit: In Ubuntu 14, there was a package percona-xtradb-cluster-galera-2.x, but Ubuntu 18 misses such a package.
apt-cache search percona-xtradb-cluster-galera returns galera-3 and percona-galera-3 because they do both break percona-xtradb-cluster-galera-3.x accordng to the package descriptions, but there is no such package in the Ubuntu 18 repositories.

https://wiki.debian.org/Teams/MySQL/FAQ does not mention Galera.