This occurs under 2 circumstances:
One is when you turn on your Xbox and go straight to Halo 3 matchmaking it often will start off saying players failed to Load Content. The cure for this is to change to either a custom games or a forge lobby and chose a different map that came on one of the Map Packs. Then just go back to the lobby.
If that doesn't work then it simply means you haven't downloaded the Map Packs.
I had this same problem, check for a crack on the bottom of your CD if there is one you can take it to a store that will fill in the cracks. If there is no crack and you still have the problem you might want to bring it to the store that you bought it at.
Maybe your disc was a preowned and the previous owner scratched or something
That means someone in the custom games lobby does not have the map you have selected. Ask Microsoft costumer support....needs more info or your disk is scratched... I would buy a new Halo 3 there not as expansive as they used to be but it will cost like $50 unless you buy it used. I have the some problem, and I'm just wondering: can you play campaign? I can't play Valhalla or High Ground, and campaign just cut out.
get a new halo 3 disc
no you can not
you can not so buy cheapscape
you get it as a certain disc at game it has mappack on it
Load Balancer
mabeye your sever is offline
if you have a mac, it wont work, if you have a PC then you insert the disk into a drive, then the installation should load. if it doesn't then try going on My Computer, then your disk drive that you put your Halo CD in, and it should load.
Over load the map with trip mines
Load balancer