First go to a level that you can get three pistols on, such as outskirts. Go pick up a pistol. Then swith to your second weapon and pick up dual weapon, such as two smg's. Go to another pistol and press x to pick it up. Now you have a pistol in you secondary and first slots. Now you can go pick up a third pistol and set it to your left hand; press Y to do this. You can get up to 144 ammo with three 96 ammo with the two and just 49 with one.