List of Achievements for Halo 3
From Halopedia, the Halo wiki
This is a list of Halo 3 Achievements. There are a total of 49 original achievements that were shipped with the game, worth a total of 1000 Gamerscore (GS), with another 30 DLC (Downloadable Content) achievements worth 750 GS. Some achievements unlock certain armor permutations. Getting 1000/1000 GS from the original achievements will get you the katana and Security Helmet.
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.