Floppy disk reliability isn't a factor with the 286 systems mentioned. The double sided 5.25" disk problems had largely been resolved by the time of the release of the IBM PC. Earlier difficulties with disks and controllers had led a number of systems to have the option of backing up to cassette tape but that doesn't apply to the IBM PC compatibles. More recent disk production may have been problematical but no one was going to do a full system backup on floppies after 2000. A 128GB hard drive would require about 80,000 floppy disks for backup which would fill a 10' by 10' room floor to ceiling and weigh as much as a small car. Not a practical solution.
The quality of disks depended on where one bought. Preformatted disks from major companies had a 99%+ success rate. The company I worked for in the 90s bought the cheapest disks possible and had a roughly 50% failure rate. Had the purchaser been instead for whichever major brand was on sale, the price per working disk was lower than the bulk purchase and bulk purchases of quality disks would have saved a lot of money not to mention developer aggravation. Good backup software would kick out a failed disk and accept a replacement at that point of the backup cycle and other than the time wasted in waiting for the disks to be tested the entire set would complete successfully. Some backup software tried to gain a little speed by not testing and also lacked good verification tools but a backup that can't be restored is useless.
The 3 backup cycle was a concession to how systems would run. With tape, a backup with typically be run over the weekend. Next week, plug in a second tape to get a second backup. Third week, third tape, third backup. Then, because of how much tape costs and the fact that falling back to very old backup isn't much use, the first backup tape would be overwritten with a new backup. Repeat the cycle and replace worn out tapes. At any time, there would be at least two backups available to recover from. If both failed, the backup administrator would be looking for alternate employment. It is possible to do the same with floppies.
Floppy failure should not be much of a problem. If a floppy was bad at the start, backup software rejects it before writing to it. Most disks should have data integrity for years so a set could be restored. It is possible that a given disk might fail between writing and restore but some of the better floppy disk backup software would restore the rest of the disk set and flag the damaged files that couldn't be restored. Maybe a second backup or install disks for programs could be used to fill in the files that failed to restore. No backup procedure can ensure 100% success but it was possible to get a very high success rate affordably with just a little care.