Short: At 200A each, I don’t think you need 3. I'd say get 2, 200A alternators and a 4 farads worth of capacitors. You also may want to consider a very nice battery with lots of amp hours. More caps if you can afford it so your lights wont dim when you show off (even more street credit) If you really have cash to burn, go ahead and get the 3 alternators + 5 caps... It can't hurt
Long explanation + crap you may not care about
Your formula is correct, however remember that your alternator will not produce full output until it reaches a certain RPM. A 200 amp alternator may not produce more than 100 amps at engine idle, if even that much. (I'm not sure on the exact figures but it's *significantly* less than the rated output.) Now, think of how often you are revving your engine compared to just driving along. Not too much juice huh?
To counter that problem, you wont be drawing that amount of power (aprox: 4000-4500W, accounting for amplifier inefficiency) all of the time. The amplifiers are capable of producing that output, but it doesn’t draw that amount of power all of the time unless it is driven to the max. For one, I doubt your going to have the volume CRANKED all the way up all of the time anyways, only for a few seconds of a time or a few songs to impress people before it becomes... well, down right uncomfortable.
Also, most songs don't have CONSTANT bass tones, but bursts of bass. There is even less current being drawn over time when there is no bass being played, even though at some points it will be drawing upwards of 4Kw. You don’t *need* to have alternators produce the amount of power that you will be drawing, that’s also what the battery is for. The alternator + battery + caps just need to be able to keep up with the overal electrical demand.