Sounds like a fault with the manufacturing of the disc as apposed to the system dude. Could be a small but nasty scratch, a glitch in the game or a corrupt save file on your HD.
I'd try other games, if they are loading well enough it pretty much rules out the Xbox.
Start a new game or if possible start from a previous save point earlier and work your way up until this point once again, chances are it will be fine after that but if it does continue to give you hassle that rules out your HD which means the disc is the problem and you simply get it replaced from where you purchased it
Also for any 360 owners, dont carry your system with a disc in it. Apparently the reader is very close to the disc and tends to leave scratches. I cannot confirm this (nor want to), just passing on what I've been told.
Oh and GHTP3 is well into development for 360. From all accounts on the official forums the game is using real time physics (i.e. bounces, swing balls etc), motion capture, a new scenario format (so thats why Greg was playing smeg loads of scenario last year
) plus much more