The first two movies are good in 3D as well. The 3D feels natural, and it's impressive in many places despite it not originally being made with 3D in mind. The 3D "director" chose to limit depth in indoor scenes to give a sense of safety, and conversely expand it when outdoors heighten the sense of vulnerability. The two movies were then re-rendered to make them natively stereoscopic.
Is it worth it if you already own a version of the movies? Depends on price, how much you like the movies, how much you want to expand your 3D library, etc etc. But I don't think anyone should worry about its 3D quality.