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 Campaign on Legendary difficulty.
Earned over 15,000 points in the Campaign meta-game on the second mission.
Earned over 15,000 points in the Campaign meta-game on the first mission.
Scored a Double Kill with a single spartan laser shot in a ranked free for all playlist.
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.
Boarded the same vehicle within 10 seconds after being boarded in any free for all playlist.
Accessed all Terminals in the Campaign.
Claimed Mythic skull.
Claimed Tilt skull.
Claimed Thunderstorm skull.
Claimed Famine skull.
Claimed Fog skull.
Claimed Catch skull.
Claimed Iron skull.
Scored 5 grenade sticks in any ranked free for all playlist.
Killed 10 enemies without dying in any ranked free for all playlist.
Promoted to Sergeant. Proven in combat, you are a Spartan.
Destroyed an enemy vehicle with equipment in a ranked playlist or in campaign.
Destroyed a banshee with the spartan laser or missile pod in a ranked playlist or in campaign.
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.
Splattered an enemy with the mongoose in a ranked free for all playlist.
Destroyed a vehicle that has three enemies in it in a ranked playlist or in campaign.
Killed 10 enemies with headshots in a ranked free for all playlist or in campaign.
Earned the MVP in a ranked playlist.
Claimed Tough Luck skull.
Killed 3 enemies within 4 seconds of one another in a ranked free for all playlist.
Claimed Black Eye skull.
Completed the Campaign on Heroic difficulty.
Completed the Campaign on Normal difficulty.
Completed the first mission of the Campaign.
Completed the third mission of the Campaign.
Completed the final mission of the Campaign.
Completed the eighth mission of the Campaign.
Completed the seventh mission of the Campaign.
Completed the sixth mission of the Campaign.
Completed the fifth mission of the Campaign.
Completed the fourth mission of the Campaign.
Completed the second mission of the Campaign.
Killed 5 enemies with the needler in a ranked free for all playlist or in campaign.
Advance to the Spartan Officer ranks (Online).
Score over 15,000 points in the Campaign meta-game on the third mission.
Score over 15,000 points in the Campaign meta-game on the fourth mission.
Score over 15,000 points in the Campaign meta-game on the fifth mission.
Score over 50,000 points in the Campaign meta-game on the sixth mission.
Score over 50,000 points in the Campaign meta-game on the seventh mission.
Score over 15,000 points in the Campaign meta-game on the eighth mission.
Score over 15,000 points in the Campaign meta-game on the final mission.
On a Legendary map, got 3 Oddball melee kills during a ranked or social match.
On a Legendary map, got a Splatter Spree during a ranked or social match.
On a Legendary map, got a flag melee kill during a ranked or social match.
On a Legendary map, got 2 flag carrier kills during a ranked or social match.
On a Legendary map, got 5 Warthog chaingun kills during a ranked or social match.
On a Legendary map, got 3 assassinations during a ranked or social match.
On a Legendary map, got two Double Kills during a ranked or social match.
After 9/25/08, completed 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 a Mythic map, infected 2 humans during a ranked or social match.
On a Mythic map, killed 2 Zombies during a ranked or social match.
On Orbital, got 2 Death From the Grave medals during a ranked or social match.
On a Mythic map, got a Bulltrue medal during a ranked or social match.
On a Mythic map, got a Perfection medal during a ranked or social match.
On a Mythic map, got a kill on another player while in monitor mode.
On Sandbox, found the hidden skull.
On Orbital, found the hidden skull.
On Assembly, found the hidden skull.
On a Mythic map, got an Extermination during a ranked or social match.
On a Mythic map, got a Killtacular during a ranked or social match.
On Assembly, got 5 Hammer kills during a ranked or social match.
On a Mythic map, got a Killjoy medal during a ranked or social match.
On Longshore, got 3 kills with a Ghost during a ranked or social match.
On Heretic, got a Sword Spree during a ranked or social match.
Found all the hidden skulls on the Mythic maps.
On Citadel, found the hidden skull.
On Longshore, found the hidden skull.
On Heretic, found the hidden skull.
On Citadel, got a Shotgun Spree during a ranked or social match.