i think..... uh...... IN A CIVIC?!?!?!?!?!?!?!?!?!
3 15's???? let me just put it his way. not only will you need ampS like max said, but you will need much dynamat(if you don't want to vibrate your car to death), 1 inch MDF, depending on which ampS you go with you will probably need another battery, maybe an alternator respin, and probably won't have a trunk when its all said and done. and if you want to hear you mid's and high's, you guessed it. replace em all and an amp for those too. hey its your money and car though. but don't expect to win any races. if you get the right stuff, i bet it'd sound pretty good though. just out of curiosity. which 15's? 3 15's in a civic. dang. not the most i've seen though. i have seen 8 12's in a 2 door cavalier. and 8 15's in a 1/2 ton chevy with no bed cut. but most impressive would have to be emma gates with 48 12's in a bronco. that is cool. and very very loud.
