If you mean Halo for the PC, there is a good site here: https://sites.google.com/site/halo1skins/hmt-3-5 If you mean for the xbox, sorry!
They are in secretive places. There are ones in Multiplayer and in Campaign. http://www.halo3planet.com/halo-3-skull-locations/ - this is a good site but you can search on youtube, they have good tutorials.
yesh complete campain on normal/heroic/legendary look on http://hubpages.com/hub/halo-3armour (a brilliant site)
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
it goes halo, halo 2, halo 3, halo 3 odst and soon halo reach will be available
depends on what you're looking for. but most good mods in halo require a jtag. but a good site to look up mods is- http://www.thetechgame.com/
Halo Games: 1. Halo: Combat Evolved (Xbox, PC) 2. Halo 2 (Xbox, PC) 3. Halo 2 Muliplayer Map Pack (Xbox) 4. Halo Triple Pack (Xbox) 5. Halo 3 (Xbox 360) 6. Halo 3 O.D.S.T (Xbox 360) 7. Halo reach (Xbox 30) Other Halo Games: 1. Halo Zero (PC) 2. Halo: Custom Edition (PC) Future Games: 1. Halo Wars (Xbox 360) 2. Halo: Chronicles (Xbox 360) 3. Halo 3: Recon (Xbox 360)4. Halo 3: ODST (Xbox 360)
The names of all the Halo games are (in order): Halo: Combat Evolved, Halo 2, Halo 3, Halo Wars, Halo 3 ODST, Halo Reach, Halo: Combat Evolved Anniversary, and Halo 4. In chronological order: Halo: Wars Halo: Reach Halo: Combat Evolved/Anniversary Halo 2 Halo 3: ODST Halo 3 Halo 4