The old-school rule of thumb for stiffening capacitors was 1 farad up to 1000 watts. So 3 farads should be good for about 3000 watts. But there are lots of other factors, such as the impedance that the amplifiers are driving and the type of capacitor (normal, hybrid, carbon, etc.). Low impedance loads, such as 1-ohm subs, will draw a lot more current from the amplifiers than a higher impedance load. If you’re really running 8000 watts, then you’ll likely need a second battery and an isolator in addition to stiffening capacitors. And 3 farad might not be enough, especially if your subs are wired to a low-impedance load. 8000 watts is an awful lot of power to be running through a car’s electrical system, so plenty of power supply upgrades are going to be needed to avoid straining the rest of the car’s electronics (and the lights).