EB wont do crap for you unless you bought some extended warranty. I will be guessing that you did not buy it, so my suggestion is to go to the xbox website and order a new copy for only $20 since it is covered by Microsoft, you will have to send in the old disc and pay the money. Within 2-4 weeks you will have a brand new halo 3 disc.
The crack is caused from your 360, it may have been bumped while you were playing causing the crack. The crack will expand into a larger one until the disc snaps. You should be able to play it still, but its your risk.