ok, my son gave me his hp lightscribe dvd640.  i already have a dvd-rom and a dvd-rw on my tower.  there is a third slot for  additional hardware.  i was told by the local pc store that i could only hook up 2 of these drives, so why is there 3 slots.  also, if it makes any difference, i already have an A drive for floppys, so the additional slot is not for that.  any advice would be helpful.