From Halopedia, the Halo wiki
Triple wielding is a glitch present in Halo 2 and Halo 3 's dual-wielding systems. The glitch allows players to use three dual-wieldable weapons at the same time; though the player cannot shoot with them all, they do get 50% more ammo, which can come very handy on big maps or on campaign maps with Flood or Hunters.
Method 1 - Exploit
This method is an exploit that allows players to triple-wield two different weapons—that is, two weapons of the same type, and an additional weapon of a different type.
- Find a weapon (Weapon A) you wish to triple wield, and pick it up. Now press Y to switch weapons, and find a different (dual-wieldable) weapon (Weapon B). Pick it up.
- You should now only have two weapons (Weapon A and Weapon B), and you should not be dual-wielding them. Furthermore, you should currently be holding Weapon B.
- Find another Weapon A, and dual-wield it with Weapon B. You should now have Weapon A as a secondary weapon, and Weapons A and B as primary weapons, dual wielded.
- Now, trade Weapon B for another Weapon A.
You have now triple-wielded. Note that with this method, you can only have dual weapons plus one in reserve.
Method 2 - Glitch
- Have two similar weapons dual wielded, and look straight down.
- Melee the ground and then quickly pick up the weapon, the corner of your screen should say you picked up the weapon, do this three times or until the screen says you have picked up the weapon three times.
- Use your newly acquired three weapons.