MariaDB Galera cluster node won't start
Please post the tail end of your MySQL error log.  Usually, it will be in /var/log/syslog or /var/log/mysql/mysql.log (depending on your setup).

Without having some details, it is hard to say, but it almost sounds like you have network connectivity problems.  Please ensure that the necessary ports are open on the third node to properly allow for all Galera traffic.