the ability to display different things on each screen will depend on what you use for your computer.
if you want to keep the smallest system possible, then you would be using onboard video (which will only have 1 video output, you can "split" the signal out with a VGA splitter but both screens will display the same thing).
to make each display separate, you will need a video card with multiple display outputs. most video cards have 2 [DVI or VGA] outputs and 1 [S-Video] output, there are physically 3 outputs but you can only use 2 of them at the same time.
a 2 output video card will be ok for you as long as the two headrest monitors can display the same thing. in this case you would hook your front monitor to one output, and hook the two headrest monitors to the other output (via a splitter).
if you need all three monitors to be able to display separately, then you will either have to buy a triple or quad output video card (they are expensive) or if your system supports it, add a second [PCI] video card to give you another one or two outputs. another option would be to hook the rear monitors up to a DVD head unit (not attached to the PC), as long as the headrest monitors have multiple inputs, you can switch them back and forth between the PC and the HU.
hope this helps, good luck