MSoft support will probably repair your 360 at no cost. That's all that they can really do I believe if your unit is still less than a year old.
I had the same problem. 3 red lights means that there is a hardware failure. And if your Power Brick is green, you have taken your Hard Drive out, and you are still getting 3 flashing red lights, there's not much left to do other than to get it sent in for repairs. And you'll be without a console for 2 weeks while it gets fixed.
Sorry man. Same thing happened to me 5 days in. I didn't really want to deal with it anymore so I got a refund.