Backup in high-availability clustering has to be handled cautiously: Even if you have set up a High Availability Setup to prevent any outages, you should always take full-out scenario into consideration. In this case, and only in this case, you will have to recover your cluster from a backup. From operational perspective, it might make sense to decide to take backups only from node 3 (which is designed to be at a disaster recovery site off-location) to reduce load and network traffic on the nodes at the main site.
We recommend setting up an automated backup schedule on all of your nodes to make sure to be able to recover everything, out of every situation, even if perhaps a failure takes a long time to be discovered.
Generally, a backup always contains all information of a cluster node (configuration and database), including its node identity. For example, a backup file taken from node 3 will not just create any node of a cluster, but exactly node 3 when restored.