Hey so here is a tut on how to mod halo 3:
http://www.ibotmodz.net/forum/topic/11364_halo_3_modding_tut/ Lol too much work typing and explaining the whole thing. This tut does a good job of it though. With halo 3 modding there a bunch of things you can do, its fun and more things are being discovered about it.
Btw if you don't want to mod, just play on a few modded maps and crap my gt is ReconIII