score:6
The date on that Air Train map is July 28, 2008. Also, the map is hosted on the site of NJ Transit, which doesn't run the Air Train. The Air Train is a service of the Port Authority of New York and New Jersey. See http://www.panynj.gov/airports/ewr-car-rental.html.
That site indicates that Enterprise is at P3, as do Google Maps and the Enterprise site itself.
The Orbitz text about the car being at a terminal with a shuttle is probably a standard text that is used at many airports and that doesn't take into account the difference between a van shuttle and the Air Train.
Of the five sources of information, the most reliable and up to date are the Port Authority site, Google Maps, and the Enterprise site. Since they also agree with each other, it is safe to conclude that the Enterprise office is at Air Train station P3.