Sadly that setup is not particularly fault tolerant. A ransomware attack will take out your primary copy and both backups, if the backup is turned on. A controller failure in the housing will take out both backups. An external force (e.g. fire) will also take out the primary and both backups. A mechanical life issue (bearing failure) will take both backups at roughly the same time.
Perhaps most likely a fat fingered user will also take out both backups in one go, or if a delete goes unnoticed everything.
See also
https://www.raidisnotabackup.com/