The rule: three copies of your data, on two different kinds of storage, with one of them somewhere else.
The original counts as one of the three. So it is your Mac, plus two backups, and one of those two lives in another building.
Three copies is about independent failure. Two copies sound like enough until you notice that the moment one fails you are down to a single copy with no margin, and that is exactly when you are also stressed and in a hurry.
Two kinds of storage is about correlated failure. Two drives from the same batch, purchased on the same day, sitting on the same desk, on the same power circuit, are not really two independent things. A power surge, a dropped bag or a firmware bug takes both.
One off-site is about the whole building. Fire, flood, burglary and a moving van each destroy every copy in the room without caring how many there were.
The most common way to have fewer copies than you think is to count a backup nobody has ever read back. A truncated write, a disk quietly failing, a sync that stopped three months ago: from the outside all of them look exactly like a healthy backup, and the count on your whiteboard says three.
The fix is unglamorous. Read the backup back, compare it against what you recorded when you wrote it, and do that on a schedule:
shasum -a 256 -c ~/backup.sha256A copy that has never been verified is not a copy. It is a hypothesis.
iCloud Drive, Dropbox and Google Drive are not backups, whatever else they are. They are mirrors. Delete a file, or encrypt it by accident, and the change propagates to every device you own within seconds. That is the feature.
They also do not give you two kinds of storage on their own: a folder still uploading is bytes on the same disk as your original, so until the upload finishes it is not a second copy at all.
All of which said, the space in those accounts is real, it is usually already paid for, and a proper backup written into a folder inside them is a perfectly good off-site copy. What matters is that a backup tool wrote it, with versions and its own integrity checks. A sync client mirroring your live files is a different thing.
The third one is the awkward one for most tools, because Time Machine cannot write to a cloud folder at
all. Anything that writes to an ordinary folder can, including rsync with an encrypted archive,
and including Copy Shelter, which encrypts before anything leaves your Mac and checks the
copy afterwards.
It is a floor, not a ceiling, and it dates from a time when off-site meant driving tapes somewhere. Two things have changed since. Off-site is now cheap enough that some people run two of them. And ransomware made a fourth property matter: at least one copy that cannot be overwritten by the machine being backed up. An external drive that is only plugged in during a backup satisfies that, cheaply.
Yes. The rule counts copies of the data, and the live one on your Mac is a copy. So you need two backups, not three.
Strictly, no: same technology, often the same batch, usually the same room. In practice two drives are much better than one, and if that is what you have, use it. Just do not tell yourself it is 3-2-1.
Not on its own. Syncing propagates deletions and mistakes, so it does not protect you from the most common cause of data loss, which is you. A real backup written into an iCloud Drive folder is a different thing, and that does count.
As often as you can stand. The question is how much work you would be willing to redo. For most people daily is right, and the off-site copy is the one that most often quietly stops, so it is the one worth checking.
By reading each of them back. Copy Shelter counts only the copies it has actually checked, and names the drive that is not plugged in.
Next: how to check a backup is actually readable.