I backup my /home folder on my PC to my NAS using restic (used to use borg, but restic is more flexible). I backup somewhat important data to an external SSD on a weekly basis and very important data to cloud storage on a nightly basis. I don't backup my *arr media at all (unless you count the automated snapshots on my NAS), as it's not really important to me and can simply be redownloaded in most cases.
So I don't and wouldn't apply the 321 rule to all data as it's simply too expensive for the amount of data I have and it'd take months to upload with my non-fiber internet connection. But you should definitely apply it to data that's important to you.