=)
MSXL: The Last Honest Chapter
Try clearing the cache on your Xbox:
* Go to the Xbox Dashboard, select the System Blade, and select Memory.
* Highlight the HD symbol and press Y.
* Press X, X, Left Bumper, Right Bumper, X, X.
Note that this will delete all Live updates to downloaded games on your console, so you will have to re-download the updates before you play the games after.
After doing this, I would recommend launch a game by yourself in each Halo 3 map to load it into the system cache again, and then try to join a game.