3 subs, different airspace

This is not a good idea, it is recommended that yes all the subs have the same power from the same amp, or even use the same airspace. I prefer using separate chambers for my subs as if there is a failure of one sub, at least I can still listen to my tunes while the other sub is sent away. My suggestion for you is this. Build the box as one large box for now, glass away on the sides as much as you like, matter of fact, I would resin the entire inside of this box (make sure to do this). Once this is done, take it out of the vehicle and fill it with water, this does two things, if water leakes, air leaks. The second thing that can be done is to calculate airspace. Once you have the proper airspace calculated, the gross volume, you can subtract from that the cubic volume of the dividers you will put in to separate the chambers, this will be the net volume. If the center box was going to be a basic cube or rectangle this is very easy, take the net cubic volume and divide by 3. Make the center chamber to the volume requirement of that last calculation. This should get you started on the right path, confused? post it on up and someone will answer it.

For the math

l x w x h = cubic inches = cubic foot

12 x 12 x 12 = 1728 cu.in. = 1 cu.ft

28.32 litres of water = 1728 cu.in. = 1 cu.ft.