Uhm... Interesting, let's see.. Every time you buy a SD card the card should come with an adapter to use the SD card with computers and other devices. Take one of the damaged cards and try to access it with a computer, does it appears as damaged? Can you format it and try it for few days NOT on the phone?
If the card works good with other devices but not the mobile then the issue can be in the os, during the boot the MBR or BRs can get damaged/overwritten. Is your phone rooted? Are you using some SD optimization apps? A bad os can do that too (bad init.sh) so the possible e solution is to remove all the optimization apps that could damage the file system or performing a software repair.
If the SD card instead don't work either with computers or other devices, try another SD card, if also a 3rd one fails then for sure it's something related to your phone and only Sony Local Support can check what's wrong, if then the new SD card won't fail.. Eh... Can happen!! Bad batches of SD cards are everywhere.
Update: Now that I think well you may have an app that try to write to SD card continously. That can damage the SD card as the SD its not meant to be used as a regular read/write storage unit.
