Add this to my.cnf for all nodes:
[mysql] binlog_format=ROW wsrep_cluster_name=clustername wsrep_provider=/usr/lib/libgalera_smm.so wsrep_slave_threads=2 wsrep_sst_method=rsync innodb_locks_unsafe_for_binlog=1 innodb_autoinc_lock_mode=2
In addition, add this to the primary node:
wsrep_node_name=node1 wsrep_cluster_address=gcomm://
Second node:
wsrep_node_name=node2 wsrep_cluster_address=gcomm://node1.server.com
Third node:
wsrep_node_name=node2 wsrep_cluster_address=gcomm://node1.server.com