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.
Claimed Iron skull.
Claimed Black Eye skull.
Claimed Tough Luck skull.
Claimed Catch skull.
Claimed Fog skull.
Claimed Famine skull.
Claimed Thunderstorm skull.
Claimed Tilt skull.
Claimed Mythic skull.
Destroyed a banshee with the spartan laser or missile pod in a ranked playlist or in campaign.
Killed 5 enemies with the needler in a ranked free for all playlist or in campaign.
Killed 10 enemies with headshots in a ranked free for all playlist or in campaign.
Destroyed a vehicle that has three enemies in it in a ranked playlist or in campaign.
Earned over 15,000 points in the Campaign meta-game on the fourth 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 final mission.
Accessed all Terminals in the Campaign.
Earned over 50,000 points in the Campaign meta-game on the seventh mission.
Earned over 50,000 points in the Campaign meta-game on the sixth mission.
Killed 4 enemies within 4 seconds of one another in a ranked free for all playlist.
Earned over 15,000 points in the Campaign meta-game on the third mission.
Earned over 15,000 points in the Campaign meta-game on the first mission.
Completed the Campaign on Legendary difficulty.
Earned over 15,000 points in the Campaign meta-game on the second mission.
Killed 10 enemies without dying in any ranked free for all playlist.
Splattered an enemy with the mongoose in a ranked free for all playlist.
Killed 3 enemies within 4 seconds of one another in a 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.
Killed 5 enemies by melee or assassination in a ranked free for all playlist.
Graduated from basic training. Stick with it to earn your Spartan Insignia.
Earned the MVP in a ranked playlist.
Score a Double Kill with a single spartan laser shot in a ranked free for all playlist (Online).
Score 5 grenade sticks in any ranked free for all playlist (Online).
Destroy an enemy vehicle with equipment in a ranked playlist or in campaign.
Score a Triple Kill with the sword in a ranked free for all playlist (Online).
Board the same vehicle within 10 seconds after being boarded in any free for all playlist (Online).
Score over 15,000 points in the Campaign meta-game on the eighth mission.
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 two Double Kills during a ranked or social match.
On a Legendary map, got 3 assassinations during a ranked or social match.
On a Legendary map, get 3 Oddball melee kills during any ranked or social match.
On a Legendary map, get a Splatter Spree during any ranked or social match.
Entered into a ranked or social playlist with 7 EXP on the 7th of the month.
After 9/25/08, completed Halo on 4-player Legendary LIVE co-op, with Iron, and everyone in Ghosts.
Got the rank of Lieutenant in any playlist in the new EXP progression system.
On a Mythic map, got a kill on another player while in monitor mode.
On a Mythic map, got an Extermination during a ranked or social match.
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 a Mythic map, got a Killtacular during a ranked or social match.
On Sandbox, found the hidden skull.
On Orbital, found the hidden skull.
On Assembly, found the hidden skull.
On a Mythic map, got a Bulltrue medal during a ranked or social match.
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 Perfection medal during any ranked or social match.
On Heretic, got a Sword Spree during a ranked or social match.
On Citadel, got a Shotgun 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 Longshore, get 3 kills with a Ghost during any ranked or social match.