shrink ibdata1 in Galera Cluster
I don't think you have many options here. If you want to maintain high availability I'd say shutdown on node and configure it as a replication slave of one of the cluster nodes. Rebuild it from a mysqldump of the cluster node. Once slave catches up, restart it as wsrep node 1 of a new cluster and then promote it as master. Shutdown the other two nodes of the old cluster and empty datadir, and let each join one at a time to the new cluster. Others may have a better suggestion though.