Galaxy S3 just died

Hey all. This is my first post in here besides comments, but I had a Galaxy S3 and it just completely died. Turned off on me, won't charge, and won't turn back on. I've tried all the things like holding the home button, the power button and the volume up or down button and nothing. I took it to the AT&T store and basically said I was screwed. I tried different batteries and nothing helped. I've tried multiple chargers, and cords so it's not that. So I was wondering if there was anything else that I could try to make my phone come back to life. I've been using an iPhone 4 while I've been away from my S3, but the iPhone 4 is just so outdated and uses 3G that it's just slow. So any advice on things to try or what to do would help.