MMS-5650 Home theater system adapter cable

I have a Dell Optiplex GX520 and recently received from a friend a MMS-5650 Home theater surround system. My friend had this surround system connected to his Dell Dimension 8400 desktop. This surround systems main speaker has 3 connectors, black, green and yellow coming out of back to hook up to your computer. I need an adapter cable to connect all 3 to integrate 3 into 1 because my Optiplex GX520 only has one speaker out port. I know Dell makes one or did in the past because it shows some sort of adapter cable in the MMS-5650's manual to connect to my built in sound card with one speaker out port. The manual gives you 2 ways to connect this home system to your computer. One is to a sound card where all 3 connectors are utilized and the other way is to my configuration of one speaker out port with built in sound cards. Does anyone know where I can get this adapter cable? or part number on Dell's website to get it there?