You could mount a laptop monitor as a screen, but unless it is touchscreen you will also need to use an input device (keyboard, mouse, etc) to control the computer.
Screen size will matter too.. as far as placement availability.
As far as hooking up multiple internal LCD screens to one laptop, I do not think it would be possible because the laptop is only designed to run that one internal screen and probably an external VGA screen. You could however, pick up a nice lilliput touchscreen for your dash and run that off the VGA plug on the laptop and run the laptop LCD screen into the backseat for the passangers by extending the cable that goes from the laptop to the internal LCD.
The touch screen would be easier to use/control, though for safety sake.. still always pull over to change mp3s, enter GPS destinations, take a call, etc.
As a side suggestion, good working LCD screens from dead or broken laptops can be sold on ebay too. You could probably get enough cash for a nice Lilliput 8" touchscreen if you sold all three lcd screens on ebay.