In Halo 2 and Halo 3 there is a trick to have 3 of the same weapon.Here are the steps to do so.
*IM ONLY USEING THESE WEAPONS AS EXAMPLES,THIS TRICK WORKS WITH ALL DUAL-WELD WEAPONS!!!*
FIRST:Take an smg as your primary weapon,then pick up a pistol as your secondary.
SECOND biggrin ual-weld a smg with your pistol out,then when you walk on a dual-weldable smg,it will give you the option to swap the smg or the pistol if you do it right.
THIRD:Swap the pistol and you will be dual-welding 2 smg's and have anotherone as your second weapon.
AS I STATED ON THE TOP,THIS TRICK(IF YOU DID IT RIGHT)WILL WORK WITH ALL DUAL-WELDABLE WEAPONS.