Achievements (Halo 3)
From Halopedia, the Halo wiki
There are a total of 79 achievements that can be unlocked in Halo 3. 49 were shipped with the game, worth a total of 1000 Gamerscore , with another 30 downloadable content achievements worth 750 . Some achievements unlock certain armor permutations.
An additional 30 achievements, totaling 750 GS, were leaked at PAX 08. All 30 of these, worth of 750 GS, were added in Title Update 2.
*Bungie later announced in a weekly update that, to receive this achievement, you actually have to reach 7 EXP on the 7th of any month, not enter with 7 EXP.