ok personally how much room you got behind you? because remember if you end up having to go sealed when getting a third one you might as well just stick ported with an upgraded amp. Other thing is its much less taxing on your electrical system to run a 1k or 1.2k and use ALL the power or if you go the 3 sub, really to push subs fully rms you need a 2k amp that you won't even be using all of so taxing elecrical system more than you need. Also if you just upgrade to a 1.2k amp class d you prob won't put much more strain on your system than your t400 because its a class a/b so draws more current than class d's.
Plus with one amp you can find 1k-1.2k amps for anywere from 150 -300 dollars. With other route factor in price of new box (since 3 subs have to be custom built by you or someone else) If your not running 0/1 gauge wire you should or at least 2 gauge to run the higher amp, then the price of a third p3, and finally an amp that will cost you over 300 to push everything acurately?
Have you really factored in all those thing you need to upgrade and the performance gained by all that money versus just buy a 150-300 dollar amp and gaining at least twice the output, and i dont know if the box is a prefab or not but if it is a good custom one would make a big diff too and you would still be way cheaper.
Sorry that was really long