i am VERY sorry to inform you of this, but you cant. in halo 3, halo reach, and halo 3: complete multiplayer experience you can.
you cant
The map, Foundation, does not exist in Halo 3 and is only available in Halo 2.
Yes, The Pit is a map that comes on disc for Halo 3.
No
muythic map pack. that comes with HALO 3: ODST
Every map in halo 3 is a good map. I perfer guardian or Blackout
You cannot make your own maps in Halo: ODST, it doesn't contain forge world. Halo 3, Halo: Reach and Halo 4 all contain forge world.
there are a number of ways to get the halo 3 recon armour variant make a cool halo 3 movie make a creally cool or funny screenshot make a cool map and upload it to your fileshare or complete the vidmaster achievements
a new map or map modified on forge? if you mean a forge made map, make an account on bungie.net and they have a bunch of map variants on their site
Yep.
If you mean making your own map, it's Forge mode.