To the O.P., a woofer needs to have space to displace air. The larger the surface area of the woofer, the more displacement it will naturally need. You're talking about 9.5 ft^2 of area just for the woofers alone leaving very little room for displacement with three 12 inch woofers. With tens, you're talking about 7.2 ft^2 of space which leaves that extra room for displacement which will result in a better, lower frequency response.
On a completely unrelated note, I've worked in consumer electronics for eight years now. I've sold home theater systems totaling up to 83,000 dollars. Every now and again we get the imbecile that decides that they know what they want but that they want to ask a sales associate anyway. These are generally the idiots that walk in there, waste your time and then ask, "Well what about the Bose?" You can break down all the statistics in the world for them and even play an audible demo for them proving that there is superior equipment out there, but their minds are already made up. Sure, I'll sell it to them; it's no sweat off my back that they are getting inferior equipment out there for a higher cost particularly given their lack of etiquette, but they have no right to complain when their outputs are outdated one month from now and replacing it would require replacing the entire thing is inundated or when one speaker blows and the company is forcing them to buy a whole new system.
What does this have to do with any of this? Right now, you are being that customer. The people here have post totals in the thousands and are considerably more well versed than you are in the field yet you continue to insist that you know what is best. Your own naivety annoys them not just because you're questioning them, but also shooting yourself in the foot. The people here are obviously passionate in what they do while you're a novice in the field. It would be best for you to heed their recommendations and go with smaller drivers with more room for displacement.
That being said, I am in a similar predicament. I just bought a 2011 Ford Ranger and am looking to put a subwoofer or two under the seat or seats. Size is irrelevant, I just want it to sound the best it possibly can. I can theoretically spend up to 800 dollars and have access to discounts with the likes of Kicker, Infinity/J.B.L. and Kenwood, though I'm certainly not going to limit myself to those brands. I already have 6.5" Infinity Kappa Perfects for the front and the rears being driven by an Infinity Kappa Four. I have the Alpine INA-W910 for a headunit.
