For clarity, the recommendation is specifically 3 copies of your data, not 3 backups.

3-2-1 backup; 3 copies of the data, 2 types of storage devices, 1 off-site storage location.

So in a typical homelab case you would have your primary hot data, the actual device being used to create and manage that data, your desktop. You’d regularly backup that data into warm storage such as a NAS with redundancy (raid Z1, Z2, etc). Followed by regular but slower intervals of backups to a remote location, such as a duplicate NAS with a secure tunnel or even an external drive(s) sitting at a friend or family member’s house, bank vault, wherever. That would be considered cold storage (and should be automated as such if it’s constantly powered).

My own addition to this is that at least one of the hot / warm devices should be on battery backup in case of power events. I’ll always advocate that to be the primary machine but in homelab the server would be more important and the NAS would be part of that stack.

Cloud is not considered a backup unless the data owner is also the storage owner, for general reliability reasons related to control over the system and storage. Cloud is, however, a reasonable temporary storage for moves and transfers.