Depends on what level You beat it on.
Gamerscore
depends on your skill
You get it by beating the halo 3 campain on legendary 9 times
you buy them on xbl marketplace, it has nothing to do with beating it.
when get achievements (like beating the game on Legendary or finding all the skulls) you can unlock some of the armor.
You do not have to do them in order, but you must complete them all on heroic (or legendary) to get the acheivement.
beating the campaigne on legendary is a fast way of unlocking most the armor. some achievemnets also unlocks armor.
Beating halo on legendary
halo 3 obviously
Halo 3: ODST is the newest of the two, but not the newest Halo game.
The halo time line goes Halo Wars, Halo: Reach, Halo:CE (halo 1), Halo 2, Halo: ODST, Halo 3. Halo 3 and ODST are practically at the same time.
After Halo 3 there was halo wars, halo 3 ODST, and Halo reach. A new company (not Bungie) will be making a Halo 4 and rumor has it there will be a remake of Halo Combat Evolved.
Halo 3 ODST is not for PC but Halo 1 and maybe Halo 2-3 are