It is possible to use the variable tripleo_backup_and_restore_enable_snapshots: false to take the backup of the 3 controllers at the same time. This functionality is not working correctly because  https://github.com/openstack-archive/tripleo-ansible/blob/stable/wallaby/tripleo_ansible/playbooks/cli-overcloud-backup.yaml#L59 

This bug was discovered in this bz: https://bugzilla.redhat.com/show_bug.cgi?id=2318953