When it comes to subs because of ohm loads people usually go with a even number of subs. I mean if you want 3 of them then want to but three amps to power each one of them right by all means go with it. You want a ported box too, and thats going to take a lot of room, i would think with the 18's ina ported box you might have just enough room to be able to keep your back seat, but you have to keep in mind where are you going to put all your amps and such? same dilema with 3 15's you going to have to find a spot to put those three amps.