I heard you kinda can mod the xbox360 version of FO3, its called hexing.
what the heck is hexing?
Probably involves hacking the hexadecimals in the game files. Also probably requires a mod chip. If my knowledge of other hex-editing examples is right, it's a right pain in the butt for very little gain. You could maybe change a character's inventory and add a billion stimpacks, but adding a new weapon or something wouldn't work out too well.
Definitely isn't condoned by Microsoft, and hence I expect this to be locked.