Full list of all 79 Halo 3 achievements worth 1,750 gamerscore. It takes around 40-50 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 49 achievements worth 1,000 Gamerscore, and there are 4 DLC packs containing 30 achievements worth 750 Gamerscore.
Completed the first mission of the Campaign.
Completed the second mission of the Campaign.
Completed the third mission of the Campaign.
Completed the fourth mission of the Campaign.
Completed the fifth mission of the Campaign.
Completed the sixth mission of the Campaign.
Completed the seventh mission of the Campaign.
Completed the eighth mission of the Campaign.
Completed the final mission of the Campaign.
Completed the Campaign on Normal difficulty.
Completed the Campaign on Heroic difficulty.
Completed the Campaign on Legendary difficulty.
Claimed Iron skull.
Claimed Black Eye skull.
Claimed Fog skull.
Claimed Mythic skull.
Destroyed a vehicle that has three enemies in it in a ranked playlist or in campaign.
Accessed all Terminals in the Campaign.
Earned over 15,000 points in the Campaign meta-game on the first mission.
Earned over 15,000 points in the Campaign meta-game on the second mission.
Earned over 50,000 points in the Campaign meta-game on the sixth mission.
Earned over 50,000 points in the Campaign meta-game on the seventh mission.
Earned over 15,000 points in the Campaign meta-game on the final mission.
Scored a Double Kill with a single spartan laser shot in a ranked free for all playlist.
Boarded the same vehicle within 10 seconds after being boarded in any free for all playlist.
Splattered an enemy with the mongoose in a ranked free for all playlist.
Earned over 15,000 points in the Campaign meta-game on the eighth mission.
Earned over 15,000 points in the Campaign meta-game on the fifth mission.
Earned over 15,000 points in the Campaign meta-game on the third mission.
Killed 10 enemies with headshots in a ranked free for all playlist or in campaign.
Destroyed an enemy vehicle with equipment in a ranked playlist or in campaign.
Earned over 15,000 points in the Campaign meta-game on the fourth mission.
Scored a Triple Kill with the sword in a ranked free for all playlist.
Killed 4 enemies within 4 seconds of one another in a ranked free for all playlist.
Killed 10 enemies without dying in any ranked free for all playlist.
Killed 3 enemies within 4 seconds of one another in a ranked free for all playlist.
Destroyed a banshee with the spartan laser or missile pod in a ranked playlist or in campaign.
Scored 5 grenade sticks in any ranked free for all playlist.
True veteran of many battles, you are a Spartan Officer.
Promoted to Sergeant. Proven in combat, you are a Spartan.
Earned the MVP in a ranked playlist.
Graduated from basic training. Stick with it to earn your Spartan Insignia.
Killed 5 enemies by melee or assassination in a ranked free for all playlist.
Claimed Tilt skull.
Claimed Thunderstorm skull.
Killed 5 enemies with the needler in a ranked free for all playlist or in campaign.
Claimed Famine skull.
Claimed Catch skull.
Claimed Tough Luck skull.
On a Legendary map, got 2 flag carrier kills during a ranked or social match.
On a Legendary map, get two Double Kills during any ranked or social match.
On a Legendary map, get 3 Oddball melee kills during any ranked or social match.
On a Legendary map, get 3 assassinations during any ranked or social match.
On a Legendary map, get a flag melee kill during any ranked or social match.
On a Legendary map, get 5 Warthog chaingun kills during any ranked or social match.
On a Legendary map, get a Splatter Spree during any ranked or social match.
After 9/25/08, complete Halo on 4-player Legendary LIVE co-op, with Iron, and everyone in Ghosts.
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.
On Assembly, found the hidden skull.
On a Mythic map, got a kill on another player while in monitor mode.
On Orbital, found the hidden skull.
On a Mythic map, got a Killjoy medal during a ranked or social match.
On Assembly, get 5 Hammer kills during any ranked or social match.
On Orbital, get 2 Death From the Grave medals during any ranked or social match.
On a Mythic map, get a Bulltrue medal during any ranked or social match.
On a Mythic map, get a Killtacular during any ranked or social match.
On a Mythic map, get an Extermination during any ranked or social match.
On a Mythic map, get a Perfection medal during any ranked or social match.
On a Mythic map, infect 2 humans during any ranked or social match.
On a Mythic map, kill 2 Zombies during any ranked or social match.
On Sandbox, find the hidden skull.
On Citadel, found the hidden skull.
On Heretic, found the hidden skull.
On Longshore, found the hidden skull.
On Citadel, get a Shotgun Spree during any ranked or social match.
On Heretic, get a Sword Spree during any ranked or social match.
On Longshore, get 3 kills with a Ghost during any ranked or social match.
Find all the hidden skulls on the Mythic maps.