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Does this make sense?
I don't think this meets the definition of 3-2-1. Which isn't a problem if it meets your requirements. Hell, I do something similar for my stuff. I have my primary NAS backed up to a secondary NAS. Both have BTRFS snapshots enabled, but the secondary has a longer retention period for snapshots. (One month vs one week). Then I have my secondary NAS mirrored to a NAS at my friends house for an offsite backup.
This is more of a 4-1-1 format.
But 3-2-1 is supposed to be:
Three total copies of the data. Snapshots don't count here, but the live data does.
On two different types of media. I.e. one backup on HDD and another on optical media or tape.
With at least one backup stored off site.
I've always understood 2 as 2 physically different media - i.e., copies in different folders or partitions of the same disk is not enough to protect against failure of that disk, but a copy on a different disk does. Ideally 2 physically different systems, so failure/fire in the primary system won't corrupt/damage the backup.
Used to be that HDDs were expensive and using them as backup media would have been economically crazy, so most systems evolved backup media to be slower and cheaper. The main thing is that having /home/user/critical, /home/user/critical-backup, and /home/user/critical-backup2 satisfies 3 copies, but not 2 media.
Hm I wonder why snapshots wouldn't satisfy 3. Copies on the same disk like /file, /backup1/file, /backup2/file should satisfy 3. Why wouldn't snapshots be equivalent if 3 doesn't guard against filesystem or hardware failure? Just thinking and curious to see opinion.
If I'm reading your example right, I don't think that would satisfy three either. Three copies of the data on the same filesystem or even the same system doesn't satisfy the "three backups" rule. Because the only thing you're really protecting against is maybe user error. I.e. accidental deletion or modification. You're not protecting against filesystem corruption or system failure.
For a (little bit hyperbolic) example, if you put the system that has your live data on it through a wood chipper, could you use one of the other copies to recover your critical data? If yes, it counts. If no, it doesn't.
Snapshots have the same issue, because at the root a snapshot is just an additional copy of the data. There's additional automation, deduplication, and other features baked into the snapshot process but it's basically just a fancy copy function.
Edit: all of the above is also why the saying "RAID is not a backup" holds true.
Right so I guess the question of 3 is whether it means 3 backups or 3 copies. If we take it literally - 3 copies, then it does protect from user error only. If 3 backups, it protects against hardware failure too.
E: Seagate calls them copies and explicitly says the implementer can choose how the copies are distributed across the 2 media. The woodchipper scenario would be handled by the 2 media requirement.