what is a big 3?

Im rewiring my car with a larger nator at the moment. It is a dual duty track car and i plan on making it pretty comfy on the street too. Since i need to run a new battery cable due to the battery disconnect, i was wondering if i should beef up the battery to chassis, engine to chassis, battery to alternator wires.

Is a big 3 usually added onto the stock wiring or replacing the stock wiring? Also, what gauge should I run? Im not planning anything too crazy as that would add too much weight but i do want a decent system in there

the alternator im running is 100A for the std one and i see 180 and 200A HO ones online.