At one ohm at full tilt it would be doing 3k rms. Assuming your alternator charges at 14.4 volts, 3000/14.4=208 amps. So at full power you would need to be able to support 208 amps. However you won't always be running the amp at full tilt, unless you play your stereo wide open all the time. So daily use would be less than 208 amps. A strong battery up front as well as one in the rear, close to the amp, and then a good high output alternator would do the trick. Get a 250 amp alt, give or take and it should be able to keep your batteries charged. Additional batteries and a larger alt would both help in keeping your voltage as close to 14.4 volts as possible.
Oh, and then you would actually need to factor in efficiency of the amp. I don't know how efficient DD amps are, at one ohm, but lets assume they're 80%. At 3000 rms, full tilt, 3000/0.80= 3750. 3750/14.4=260. So at full power, after considering efficiency, you would need to be able to support 260 amps.
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