Return of the Jedi is a notable exception!
But I suspect any number of sequels (not only IIIs) fall short because they try too hard to reinvent the story to make it interesting. The effect might be more pronounced for IIIs because at that point it isn't just about a great original story but more about getting something/anything up on the screen to make money for the producers. By the time of IIIs, the original writer and director might be gone as well as many others initially involved.