No problem, if you do enough reading through this website you'll find out all you need to know.
Oh and don't run a 1200WRMS amp on a stock electrical. Look to upgrade your electrical as well.
Yeah, you'll need to step your game up on amplifiers to handle the ohm load though:
Theirs a couple of brands you can choose from, maybe the Sundown-1200D but you'll need electrical upgrades on your car
I didn't mean to bring you down or anything, it's like this on car audio man, you get what you pay for. Kicker is great starter brand, definately better than Kenwood and they're not expensive. (not too much depending on your budget)
Well, unless you're getting a 450W RMS for that 3rd sub, it's gonna get less or more than the rest of the subs depending on the amp you get. You really want that?
And why you so set on different enclosures?