No respectable shop will install it for you. The NB was not designed for the middle seatbelt. It was not tested with a middle seat belt, and no shops that I know of are willing to take on that much liability were something to happen to a person belted into the middle seat.
i really wasnt even considering sitting three people, i just thought it would be nice to have a 60/40 split for longer objects and still have a baby seat. but as it was said, 3 inches might as well be 3 feet.