Yes, you do.
yes
you get it as a certain disc at game it has mappack on it
it depends if you have halo Reach, 3 or ODST. but if you have Reach you should just have to select it from your hard drive storage and play it. To get the gamemode it usually comes with the map.
Yes
Halo 3 cannot be played on a PC, or any other device but an Xbox 360. It can be installed to the hard drive of an Xbox, but it is strongly recommended you do not as it will drastically increase load times.
I think that the disc for halo 3 you have is scrated up, so you can't load the maps. A scratched disk wouldn't stop you from playing Rats Nest because it is a DLC so either Something happened while downloading or your hard drive is messed up.
You need a XSATA Hookup/cable or an Xplorer 360 Hookup/cable.
if you mean to "unlock" them...no. You have to purchase and download those maps. They simply arent on the game disc and need to be installed on the hard drive. You also could buy ODST, and that includes a second game disc that has everything Halo 3: Multiplayer on it.<-----thats they best deal, plus you get Halo: Reach multiplayer beta
Because when you have been playing for too long your hard drive overheats if that happens then you need to switch your xbox off ASAP
No. Anniversary only comes with the six new matchmaking maps and the new firefight map. You only have access to those maps on the Anniversary disc. The only way to play both the Reach maps and the Anniversary maps is to buy the map pack for Anniversary or buy the game and use the code inside the case to upload the Anniversary maps to your hard drive for your Reach disc to access.
You can't. The Mythic Maps our only playable on Halo 3, not Halo Wars.