Purchase the guns you want to use from the Item Shop.

Go into a game then select the weapon you want to start with from your inventory, then press OK.

Before you hear the voice sound "GO! GO! GO!", quickly select another gun from your inventory before the game starts, this gun will be your extra gun it can either be a pistol or rifle.

If your timing is correct you should have (three) guns instead of just two.

Move your mouse scrolling wheel (up and down) during gameplay, if you don't notice any weapons show up on the game screen then you did the cheat right.

Press the buttens 2 or 3 to pull out either your pistol or rifle. You can't play your third gun all the time just when you first pulled it out, you can't bring it back again unless you repeat the glitch.