Halo 3/Armor

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.

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
Katana - You must unlock 49 out of the 49 achievements.
 * 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.

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. (Helmet is currently Glitched, no EXACT way of getting it.)
 * 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.

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 Armor

 * 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.