Keep 3 copies of anything that matters, on 2 different media, with 1 off-site.
Each rule breaks a different failure mode: device loss, media-type failure, and site-level disasters (fire, theft, flood).
Three copies of a file that was already corrupt is three copies of a corrupt file. The rule counts copies and says nothing about verification or restore testing — an untested off-site backup has failed at the only moment it matters, and nobody finds out until then.
Keep 3 copies of anything that matters, on 2 different media, with 1 off-site. Each rule breaks a different failure mode: device loss, media-type failure, and site-level disasters (fire, theft, flood).