this isn't, or shouldn't be a "common problem" - I have a bunch of these drives, some bought, some freebie handouts from trade shows, etc., and in 3 years, I haven't had a failure. So three different drives all failing suggests a problem somewhere in your environment.
At risk of "teaching grandma to suck eggs", you are handling these things correctly, I assume. Like not just pulling the things out. On a Windows system ALWAYS use the green icon in the task bar to stop the drive and wait for the confirmation that it is safe to remove the drive. This will ensure all files are closed and buffers flushed. If you don't get the confirmation message, leave the drive in place until you shut the computer down. Any other method risks corrupting the data or worse, the FAT.
Don't defrag these drives. The drives do have a finite number of write cycles and by default, the drives will scatter the writes across the whole drive. By defragging, you move the date back to the starting sectors and you will wear them out faster. Fragmented flash drives do not have the performance problems associated with spinning disks.
Looking though your list of potential damages that you know haven't happened, the only omission I can see is a static discharge. Most flash drives, at least the metal cased ones, are reasonably well protected but a static discharge could affect the drive if it hit the connector. Try not to touch the actual connector.
I've assumed that the cables from the motherboard on your PCs to the external ports are correctly wired - it is possible on some boards to plug the connector in the wrong way round - most plugs have pin 1 marked in some way. Maybe worth a check.
Is the data lost forever? Maybe or maybe not. If it was data corruption on write, you might expect only the file you were writing to be affected but not necessarily so. To write a block, these drives read the whole of the sector in which the block is to be written, erase the whole sector and write the sector back with the new data added to the existing data, so some older files may be affected.
There are specialist data recovery companies that may be able to retrieve your data but these can be expensive. One of the better ones here in the UK is Kroll Ontrack. Their web site has trial downloads that you can use to look at the drive and see whether anything can be recovered. If so, you can buy the package to do it yourself. If the trial doesn't show up anything, they do have a free call and quote service. During normal business hours, one of their techies gets back to you within a couple of hours to discuss your problem. If they think the data can be recovered, they will give you a no obligation quote for their services. They may operate in other countries too or there will be equivalents.