The Vidmaster Challenges Title update for Halo 3 has 3 achievements worth 0 gamerscore.
Get to the rank of Lieutenant in any playlist in the new EXP progression system.
Enter into any ranked or social playlist with 7 EXP on the 7th of the month.
After 9/25/08, complete Halo on 4-player Legendary LIVE co-op, with Iron, and everyone in Ghosts.