Most likely because your disc is damaged or dirty. You can try cleaning it by following these instructions:
If the maps still won't load, your disc is damaged and you will need a new one.
I pretty sure there is no 3rd player for halo campaign.
no halo "campaign" is a single player game
EDITING... From Tony: you have to be 2nd player in campaign
yes
Finish the Halo 2 single player campaign.
Be player 2 in the halo 3 co-op campaign. Or, play the Arbiter levels in halo 2.
Everything in Halo 4 as of now is co-op. Campaign, Spartan - Ops, and Infinity (Multiplayer) Internet, local, and system link.
Halo Reach is definitely for single player. Campaign and Forge, you can do alone, but you will get the most amount of entertainment out of the multiplayer modes.
you can if you play coop campaign mode. The second player will be the arbiter
Yes, there is a multi-player (odst) campaign and a new HALO 3 disc with a muti-player campaign
In my opinion, Halo 3, because the storyline is better (partially because it continuing the story from 1 and 2). Halo Reach's campaign is still good, but should've been a bit longer. Anyway, don't let me decide for you, make your own decision :)
I don't think there is. You can only play campaign and the player vs. player modes.