I've never heard of a 3 1/2" floppy disk losing its formatting. Usually the disk was damaged somehow or the floppy disk drive went bad and cannot read them.
1. You are saying that you inserted floppy disks into a computer with a 3 1/2" floppy disk drive and got an error message right? What is the exact error message?
2. Did you try placing another floppy disk in the floppy disk drive and seeing if it reads that disk? Sometimes one floppy disk has a problem and the rest are fine.
3. Did you try placing one of the floppy disks into another floppy disk drive? Sometimes the floppy disks are fine but a specific floppy disk drive has stopped working and you need to use another drive.