How to Prevent the Connections for MariaDB Galera-Cluster When There is Only One Active Node?

I was using "rsync" as the wsrep_sst_method in my galera.cnf files on 3-Node Galera Cluster. When it was so, the system served when there was at least 2 active node. So when any two node were gone, the active one did not serve. It was good enough but when I upgraded the system and assigned the wsre_sst_method = mariabackup, it is serving if there is any active node. So when any of two nodes are gone, it is not preventing the working of the last active node. I dont want system works when there is only one active node. I want the system work like before upgrading without changing the wsrep_sst_method. So the problem is simple. How can I prevent the system serving when there is just one active node?