I don't have X-box live because i bought a banned console since it was a lot cheaper. Although now that I've seen all these maps i really wish i would be able to install them.(not the bungie made ones like sandbox i have those from halo ODST mythic pack but community forged ones). I have so far been unable to find any way to do thus and was wondering if it is possible to do so.
You have to be online on Halo 3 to download a Map, since you can't be online there is no way for you to download them. You could take your account to a friends place and download them there, I can't think of another way.
Take your harddrive to another persons house, that way when it downloads it will save on your HD ♥ Loopholes ftw.