From Halopedia, the Halo wiki
Were you looking for the Halo: Combat Evolved glitch that allowed the player to carry three weapons?
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.
You have now triple-wielded. Note that with this method, you can only have dual weapons plus one in reserve.
Method 2 - Glitch