I never had any luck with the online ones to be honest. Got burned twice. Definitely sounds like the cards. How old are they though, and how used are they? They can only be written to or overwritten an undefined, finite number of times and they will crap out on you.
Yup, I had the same problem. Bought a 32 gig class 10 sandisk from amazon. Worked about a month, then crapped out. Sent it back to amazon no problems. I got a replacement from Best Buy, hopefully it will last long term.
I had a Costco Sandisk 32gig HC go bad on me. I'll try Costco for a replacement first. If not there, then I'll contact Sandisk.
I just ordered a Sandisk 64gig XC card that shouldn't require formatting in the GS3. We'll see how that works.
EDIT: I called Sandisk Tech support @ 866-726-3475 and got an RMA#.
They're sending me a new card.