get an xbox live gold account, connect to the net, and play.
Halo 3 does work on Campaign as well as Xbox Live with up to 4 people
No it will only work on Xbox 360. Halo 1 and 2 will work on the original Xbox.
no, you must connect to xbox live to download halo 3 map packs.
no no Xbox 360 games work on the normal xbox
obviously
Yes, you can play firefight in Halo 3 ODST without Xbox live.
Xbox:$200 Halo 3: $50-60 i think Xbox Live:About $80 but you can get it cheaper
Yes, Halo 3 supports multiplayer Xbox Live gaming for any one with an internet connection and a xbox live Gold account
You have to download it over xbox live
Yes. An Xbox Live Gold Subscription is required to play Halo 3's multiplayer.
Yes, you have to pay for Xbox Live for any game
Yes you can, but games like Halo 3 were made to work best with a hard drive.