The Longitude has a different (swept) wing, different tail, and different type certificate and rating. Despite the same fuselage cross section, it's not the same airplane, any more than the 737 and 727 are the same airplane.

They aren't 3 different airplanes. They're variants of the same airplane, with same type rating, same wing, same tail, and the same flight characteristics. From the pilot's point of view, the only differences are some minor changes to the displays and moving a couple of switches in the cockpit to accommodate the larger displays.

Having the same subset of pilots flying the Latitude and Longitude would certainly save the company money, but would defeat the safety advantages of having the pilots fly only one type at a time.