The largest total depth of of any subwoofer enclosure I have seen for an F150 with no seat lift is 5.5" (interior clear dimension. So the first thing you need to do is find a subwoofer with a total mounting depth of 5.25" or less (top mounting depth).
That os the limiting factor, not the diameter. Ten inch subs...no doubt. But at 5.25" deep, you wont find many. At 12" diameter, you will find even less available at 5.25" deep.
Perhaps consider shallow mount subs? A friend of mine just tried the JL Thin line 13" diameters which have a mounting depth of 2.54"!!!! He is a professional competitor and claims this is the bes this car has ever sounded (with proper enclosure of course).
http://mobile.jlaudio.com/products_s...p?series_id=30
Next, you need to know, what the mfrs recommended sealed enclosure volume is for the sub you wish to use. THen compare it to the free area available in the box you choose to buy or build. You can go with a ported enclosure, but that requires larger boxes so i assume you are going sealed.
I highly doubt you can get 3 subs under the seat.
