If you're on a budget this is what I suggest you do. Start by figuering out exactly what your budget is, then add $200, if you don't have it then wait till you do. Secondly, buy used instead of new, your dollar goes a whole lot further. Listen to some of the advice you have already been given, you can build a very good system with only one sub and a good amp, check the classifieds here and canuckaudiomart.com, diymobileaudio.com they all have some good deals to be had. Design and integration of your system is more important then the hardware you select, keep this in mind when installing your equipment.

 

Notice a theme here?

1 sub > 3 subs
well built box > crappy box

Check the marketplace. Jalat has a sweet-*** 15" sub for sale and it's at a ridiculous price.

 

More is not better. Bass response is all about the proper matching of subs to box to amp, screw up on any one of those 3 and you are going to have problems. Figure out your budget and space for a box. Then I can build you up a proper system. What vehicle is it going into, is there a aftermarket or factory head unit in it, does it have a subwoofer output?