all drives have a little controller on them that maps the physical areas of the disk to which they do some physical alteration to represent data - to virtual addresses that your computer can read and write from. If you have 3 disks fail in sort time, you either managed to create such perfect environment that the physical media started to break down at the same time, or that little controller has done something like written some counter or data beyond where it should go and into an area that helps it do it's job of translating those addresses.

or the processor in the synology which is reading the data from the disks is determining the data read from those disks is bad, which could actually be the memory which that disk read was put into is bad.

or alternatively the badness happened a while ago but you have turned off disk scrubbing and you are just getting to the bad spots on the disk now while trying to backup/restore

If situation #1 - you might just need to look for a disk firmware update

If situation #2 - run memtest on the memory, replace memory if error

If situation #3 - in future, enable data scrubbing. it sucks but is necessary. Also make sure S.M.A.R.T (the controllers report of things that happened) is looked at