A new addition to Halo 3, and to the Halo series, is the ability to customize your avatar (Spartan II or Elite) with a variety of different armor types. You are able to customize your helmet, shoulders, and chest plate.
Spartan II Armor
MJOLNIR Mark VI Armor
Helmet - Available from the start
Shoulders - Available from the start
Chest - Available from the start
Close Quarters Battle Armor
Helmet - Available from the start.
Shoulders - Available from the start.
Chest - Available from the start.
Explosive Ordnance Disposal Armor
Helmet - You must complete the campaign mode on Legendary.
Shoulders - You must complete the 'The Ark' level on Legendary.
Chest - You must complete the 'Tsavo Highway' level on Legendary.
Extra Vehicular Activity Armor
Helmet - You must complete the campaign mode on Normal, Heroic, or Legendary.
Shoulders - You must complete the 'The Ark' level on Normal, Heroic, or Legendary.
Chest - You must complete the 'Tsavo Highway' level on Normal, Heroic, or Legendary.
Scout Armor
Helmet - You must unlock the 'Used Car Salesman' achievement.
Shoulders - You must unlock the 'Mongoose Mowdown' achievement.
Chest - You must unlock the 'Too Close to the Sun' achievement.
Hayabusa Armor
Helmet - You must claim all 13 hidden skulls.
Shoulders - You must claim 9 of the hidden skulls.
Chest - You must claim 5 of the hidden skulls.
Katana - You must unlock 49 out of the 49 achievements.
Mark V Armor
Helmet - You must unlock the 'USNC Spartan' achievement.
Orbital Drop Shock Trooper
Helmet - You must unlock the 'Spartan Graduate' achievement
Rouge Armor
Rogue Head - You must unlock the 'Spartan Officer' achievement.
Security Armor
Helmet - You must unlock 49 of the 49 achievements.
Shoulders - You must unlock 39 of the 49 achievements.
Recon Armor
Helmet - The Recon Armor is only available as a gift from Bungie.
Shoulders - The Recon Armor is only available as a gift from Bungie.
Chest - The Recon Armor is only available as a gift from Bungie.
Elite Armor
Combat Armor Helmet - Available from the start. Shoulders - Available from the start. Chest - Available from the start.
Assault Armor Helmet - Available from the start. Shoulders - Available from the start. Chest - Available from the start.
Flight Amor Helmet - You must complete the campaign mode on Heroic or Legendary. Shoulders - You must complete the 'The Ark' level on Heroic or Legendary. Chest - You must complete the 'Tsavi Highway' level on Heroic or Legendary.
Commando Armor Helmet - You must unlock the 'Overkill' achievement. Shoulders - You must unlock the 'Killing Frenzy' achievement. Chest - You must unlock the 'Triple Kill' achievement.
Ascetic Armor Helmet - You must unlock the 'Steppin' Razor' achievement. Shoulders - You must unlock the 'Overkill' achievement. Chest - You must unlock the 'Up Close and Personal' achievement.