One
possible explanation could also be that Oracal's
Tech Data Sheet says that material is calendared, whereas the 3M laminate you used is
cast- though that would usually cause weird shrinkage more than anything, and would (most likely) show up long before three years had passed.
Interestingly, pretty much
every media says (just like this Oracal does) that it has a seven year life-span- but that's always qualified as
unprinted. Once the ink is there, that lifespan drops and you have to do some serious digging to find info on that. In many cases, that number is three years- but it's only warranted if you use the manufacturer-spec'd laminate, inks, ink coverage, etc. If you're providing a warranty beyond that, may I suggest doing the digging for that info and making sure you're not providing a warranty that's above and beyond? Doesn't help you here, but it might in the future.
Like a few others mentioned, though, I'd think the more likely culprit is the unusual amount of rain your area has had recently causing issues that are really beyond anything anyone would expect. Hopefully the customer is reasonable if you're offering a discount on replacing the graphics. If they were already in the process of want to change the graphics when this occurred, I'd explain the "act of god" (or whatever) circumstances and sell the the change at your normal price (maybe tossing in a small discount as a good faith measure).
In any case, best of luck.