The "Big 3" usually refers to three wires in the charging system:
-the wire between the alternator and the positive battery terminal
-the wire between the negative battery terminal and the vehicle chassis
-the wire or ground strap between the engine block and the chassis
When you install a high-powered sound system, or a high-output alternator, you should consider upgrading these wires to a heavier gauge. The stock wiring is often too small to support the current requirements of the after-market accessories.
A "Big 3" upgrade can be helpful, but it's not a magic bullet. If you have an audio system that requires more average power than the stock alternator can support, this won't be fixed by changing the wiring.
