You dont really need a 3rd subwoofer, or even a second. My advice is get a good quality sub (MTX, Digital Designs, Fi) and run that off a good amp. this will save space in your boot and you wont look like such a ***** with 3 subwoofers that arnt as loud as someone elses 1 subwoofer. Just my advice. and also, do these subwoofers already come in a box or are you putting them in a custom ported enclosure?