install Galera but mariadb existing on system
Try using aptitude instead of apt-get. It is more intelligent. It not only will handle downgrading conflicting packages for you, but will make a series of recommendations asking you which of many possible suggested working scenarios you would like. e.g.

sudo aptitude install rsync galera-3 mariadb-galera-server


If you don't have aptitude on your machine yet, get it with

sudo apt-get install aptitude