(If you want peace prepare for war)
ok main thing is yes in order to do any real modding in halo 3 you have to have a jtagged xbox to run unsigned code (example aim-bot, flaming recon, and super-jump) you can run modified maps from a program not giving away name bungie dosent mind modded maps played for fun and they don't care to much about single player campaign either but as soon as you go into matchmaking you may last a few games but you will get banned but you can also unban it its just a lot of work not going into detail some games encourage modding ( fallout3,borderlands,oblivion) amongst others it gives the game new ideas and different play value for fun but they don't want you using it to cheat yourself to a higher rank are rating but anyway if your modding for fun go ahead its a interesting hobbie but don't use it to cheat and atleast beat the game with no mods before you mod