The big 3 is running larger wires on top of your stock wiring in 3 strategic locations.
1. Battery Ground to Chassis
2. Alternator to Battery Positive
3. Alternator Case to Chassis
Thus, the "Big 3"
This is done to prevent dimming and voltage drops in a system. As a side effect, your alternator will last longer under the extra strain your sound system creates.
Probably the best bang for buck $20 car upgrade I've ever done. Any wire size from 4 to 0 guage is what'd I'd sugguest.