Halo 3 Unlockable

Special Ending

If you beat the game on Legendary mode, you will get a special, extended ending. Keep in mind that you only have to beat the final stage on Legendary mode, and not necessarily the rest of the game, in order to unlock this ending.
Unlockable: Elite Armor Permutations

Unlock the following Elite armor permutations by fulfilling the requirement listed…

* Ascetic (Body) - Get the Up Close and Personal Achievement
* Ascetic (Head) - Get the Steppin' Razor Achievement
* Ascetic (Shoulders) - Get the Overkill Achievement
* Commando (Body) - Get the Triple Kill Achievement
* Commando (Head) - Get the Overkill Achievement
* Commando (Shoulders) - Get the Killing Frenzy Achivement
* Flight (Body) - Complete Tsavo Highway (on Heroic or Legendary)
* Flight (Head) - Complete the campaign mode (on Legendary or Heroic)
* Flight (Shoulders) - Complete The Ark (on Heroic or Legendary)

Spartan Armor Permutations

Unlock the following Spartan armor permutations by fulfilling the requirement listed…

* EOD (Body) - Complete Tsavo Highway (on Legendary)
* EOD (Head) - Complete the campaign mode (on Legendary)
* EOD (Shoulders) - Complete The Ark (on Legendary)
* EVA (Body) - Complete Tsavo Highway (on Normal, Heroic, or Legendary)
* EVA (Head) - Complete the campaign mode (on Normal)
* EVA (Shoulders) - Complete The Ark (on Normal, Heroic, or Legendary)
* Mark V (Head) - Get the UNSC Spartan Achievement
* ODST (Head) - Get the Spartan Graduate Achievement
* Rogue (Head) - Get the Spartan Officer Achievement
* Scout (Body) - Get the Too Close to the Sun Achievement
* Scout (Head) - Get the Used Car Salesman Achievement
* Scout (Shoulders) - Get the We're in for Some Chop Achievement
* Security (Head) - Get 1,000 Gamerscore Points
* Security (Shoulders) - Get 750 Gamerscore Points

Source: www.ign.com