Halo 3 (Xbox 360)

Second Ending
Beat the game on the hardest difficulty, Legendary mode, and you will get an extended ending. Oh yes, it's very cool. But to do this, you'd have to beat every level on Legendary, just the final one. Good luck Spartan.

Scarab Kills
If like us you don't enjoy having to ditch your vehicle to kill a Scarab - the huge four-legged Covenant spider tanks - you can kill them with brute force. On the back of every Scarab, there is a circular glowing spot you can shoot to do damage. Located below the Scarab's huge rear-mounted plasma turret, you can destroy the section of armour that protects the the critical 'soft spot'. 

Once this section of armour has been destroyed - it'll take half a dozen rockets, or a few blasts from Hornet rockets or Scorpion tank shells - you can just shoot the soft spot and its Scarab-a-bye-bye.

Plasma Pistol
One of the plasma pistols saving graces is its vehicle paralysing ability. Just charge up the pistol to full strength and fire at a tank or any enemy vehicle and it will become stunned - but that doesn't mean its weapons will stop firing.

Hint: Flood Easy Kills
The Flood take a lot of damge from spiker's melee attack. Just hack away, and some enemies will die with one well placed hit.

Hint: The Skulls!!
Skulls unlock cool stuff when you play, adding to the variety of Halo 3, and as soon as you've found a skull and collected it, you can save and quit and you'll have unlocked it. You can only claim/collect the skulls under the Normal, Heroic or Legendary difficulty settings, and you must start from the beginning of the mission to find them. You will not be able to see the skulls if you load from a checkpoint. 

In Co-op mode, as soon as one player picks the skull up, all players will receive the achievement. To use skulls, press X at the lobby screen. This will give you a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements.

Hint: Silver Skull Locations

• Blind (Located: Sierra 117)
  The arms of the player and the motion tracker disappear.

• Grunt Birthday Party (Located: Crow's Nest)
  If you manage to get a headshot on a Grunt, the Grunt will explode. One of the coolest skull skills by far.

Hint: Gold Skull Locations

• Black Eye (Located: Crow's Nest)
  When activated, you will have to battle enemies with melee tactics to regain shield energy.

• Catch (Located: The Storm)
  This skull makes enemies will throw more grenades.

• Famine (Located: The Ark)
  This skull ensures that dropped weapons will hold less ammunition.

• Fog (Located: Floodgate)
  This skull makes the motion tracker disappears.

• Iron (Located: Sierra 117)
  This skull makes sends you back to the beginning of the stage you're on when you die instead of to the last checkpoint.

• Mythic (Located: Halo)
  This skull makes all enemies have twice as much health. Makes Legendary co-op games interesting.

• Thunderstorm (Located: The Covenant)
  This skull makes enemies much tougher, and generally of a higher rank.

• Tilt (Located: Cortana)
  This skull makes enemies armour tougher, and able to withstand more weapons fire.

• Tough Luck (Located: Tsavo Highway)
  This skull makes enemies more skilled at getting away from grenades.

Spartan Armor Costumes
You can unlock new Spartan armor costumes in Halo 3 by completing these tasks:

•      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

Elite Armor Costumes
You can unlock new Elite armor costumes in Halo 3 by completing these tasks:

•      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)

Hint: Bonus Stuff #1
On the second level of the campaign, when you're escorting characters to their ships, you can head the wrong away from where you're supposed to go to complete your mission. Do this, and you'll come across two characters, split by a door and having a rather light hearted convo.

Hint: Bonus Stuff #2
During the campaign, when cortana does that weird ghost thing on the screen, crouch 10 times afterwards and if done correctly she will do a dance for you. Bungie were clearly a little bit wacked out on wowie sauce...