The 'big 3' is an upgrade of the 3 main wires in your engine bay:
Alternator + to battery +
Battery - to ground (most important by some people's accounts)
Alternator ground to ground
The stock wires are about 8 gauge or so. You upgrade these wires to 0 gauge or 4 gauge...something larger so that current will travel more easily. You DO NOT have to replace the stock wires; simply put the new ones on in addition to the stock ones. So, you'd basically have 2 runs of everything.
I had good results doing this. A lot of others have, too. Some haven't gained anything.
Overall, this is just a cheap way to help fix your problem of lights dimming, etc. You can usually get the 10-12 ft of wire needed, and a fuse and new battery terminals for less than $70 if you look around. $70 is cheaper than a $150 battery, and that's cheaper than a $250+ alternator.
In most cases the Big 3 will solve the electrical strain issue and you don't have to keep pouring more money into your car. In addition, if you do ever upgrade to a high output alternator, you'll have to upgrade the stock wiring anyway.
