you have a GM car, look closely at the wiring you will notice the alternator charging wire is coming to the starter but joined with a thinner wire in between. This is the "alternator fuse wire" - if your main wire shorts out somewhere this wire will blow/melt disconnecting the circuit.
A big three setup omits this safety and is very dangerous IMO. The alternator wire is quite thick, it wont melt quickly but can literally start a fire if dead shorted.