Here is the full list of unlockable achievements in Gears of War 3 for the Xbox 360.
Ain’t My First Rodeo (50) – Complete all campaign Acts on Hardcore Difficulty (Standard or Arcade)
All for One, One for All (10) – Earn the Bronze “Force Multiplier” medal
Anvil Gate’s Last Resort (10) – Story Progression in Act 3 Chapter 1 (Standard or Arcade)
Award Winning Tactics (25) – Earn at least one Onyx medal
Baird’s Favorite Kind of Toy (10) – Story Progression in Act 4 Chapter 5 (Standard or Arcade)
Brothers to the End (10) – Story Progression in Act 3 Chapter 5 (Standard or Arcade)
Collector (5) – Recover 5 Campaign Collectibles (any difficulty, Standard or Arcade)
Enriched and Fortified (10) – Complete all 50 waves of Horde mode (any difficulty, any map)
First Among Equals (25) – Earn the Silver “Number 1” medal
Hoarder (15) – Recover all 42 Campaign Collectibles (any difficulty, Standard or Arcade)
It’s All About the Loot! (25) – Earn the Bronze “Loot Courtesan” medal
Judge, Jury and Executioner (10) – Get a kill with every possible execution finishing move (any mode)
Lambency (50) – Execute an Epic employee, or someone who already has Lambency, in Versus multiplayer (any mode)
Level 5 (5) – Reach level 5
Level 10 (10) – Reach level 10
Level 15 (15) – Reach level 15
Level 25 (25) – Reach level 25
Level 50 (50) – Reach level 50
Look at That, Instant Summer. (10) – Story Progression in Act 5 Chapter 2 (Standard or Arcade)
Lost Your Good Driver Discount (10) – Story Progression in Act 3 Chapter 3 (Standard or Arcade)
Marcus, It’s Your Father (5) – Story Progression in Prologue (Standard or Arcade)
My Fellow Gears (50) – Complete all Campaign Acts in Co-op (any difficulty, Standard or Arcade)
My Turf! Cougars Territory! (10) – Story Progression in Act 1 Chapter 5 (Standard or Arcade)
Oh Yeah, It’s Pirate Time (10) – Story Progression in Act 2 Chapter 5 (Standard or Arcade)
Ok. Faith. Yeah. Got It. (10) – Story Progression in Act 5 Chapter 5 (Standard or Arcade)
Okay, Now We Find Hoffman (10) – Story Progression in Act 2 Chapter 1 (Standard or Arcade)
Pack Rat (10) – Recover 20 Campaign Collectibles (any difficulty, Standard or Arcade)
Putting it Scientifically… (10) – Story Progression in Act 1 Chapter 6 (Standard or Arcade)
Ready for More (50) – Complete all campaign Acts on Casual or Normal Difficulty (Standard or Arcade)
Remember the Fallen (15) – Recover all 15 COG Tags during the Campaign (any difficulty, Standard or Arcade)
Seriously 3.0 (100) – Reach level 100 and earn every Onyx medal
Socialite (70) – Earn the Onyx “War Supporter” medal
Swimmin’ in Glowie Gravy (10) – Story Progression in Act 1 Chapter 2 (Standard or Arcade)
Thanks For Flying GasBag Airways (10) – Story Progression in Act 2 Chapter 7 (Standard or Arcade)
That’s Just Crazy (75) – Complete all campaign Acts on Insane Difficulty (Standard or Arcade)
The Versus Sampler Platter (10) – Complete one match of all six Versus game modes (Standard or Casual)
Think You Can Handle That? (10) – Story Progression in Act 4 Chapter 3 (Standard or Arcade)
Wait, What Time is it? (10) – Earn the maximum Consecutive Match Bonus in Versus multiplayer (Standard or Casual)
Was it Good For You? (10) – Story Progression in Act 3 Chapter 2 (Standard or Arcade)
We Few, We Happy Few… (50) – Complete all Campaign Acts in 4 player Co-op (any difficulty, Standard or Arcade)
We Struck Gold, Son! (10) – Story Progression in Act 1 Chapter 3 (Standard or Arcade)
Welcome to Arcade Mode (10) – Complete 5 Arcade Campaign chapters in co-op (any difficulty)
Welcome to Beast Mode (10) – Survive all 12 waves of Beast mode (any difficulty, any map)
Welcome to Horde Mode (10) – Survive the first 10 waves of Horde mode (any difficulty, any map)
Welcome To -redacted- (10) – Story Progression in Act 4 Chapter 6 (Standard or Arcade)
Welcome to the Big Leagues (0) – Demonstrate your skill in Casual Versus multiplayer
Welcome to Versus (10) – Kill 10 enemies in Team Deathmatch (Standard or Casual)
Wreaking Locust Vengence (10) – Get a kill with every Locust monster in Beast mode (any difficulty)
You’re Dead! Now Stay Dead! (10) – Story Progression in Act 5 Chapter 6 (Standard or Arcade)
Here is the one secret achievement you can unlock in the game:
Respect for the Dead (5) – Your respect for the dead earned you access to Griffin’s special weapons stash
Tip: To unlock the Respect for the Dead achievement, in Act 4: Chapter 1 “Ashes to Ashes”, you must make it through the level without shooting, hitting, or otherwise touching any of the Ash Bodies.
If you do this properly, once you reach the stranded camp, the achievement will unlock, and a weapons locker will be unlocked for you.
