Most third movies tend to be the weakest.
Contrary to popular belief, the trilogy concept isn't necessarily a good one. Only a few people can pull it off. Just look at X-Men, Terminator, and Spider-man. Third movie did a number on the franchise.
Or if they're not outright bad, they're simply inferior to the sequel. Like Return of the Jedi, Toy Story 3 and the Dark Knight Rises. Again, that's not to say they're bad films, they're just not better than the first sequel, which was itself (probably) much better than the original.
And then you have a rarer few that are actually better than the first sequel. Some because all the movies were good, like Return of the King. And some because the first sequel was inferior, like Indiana Jones and the Last Crusade and Revenge of the Sith (not that it's a good movie, but it sure was better than Attack of the Clones). Those are rare though.
Obviously your mileage may vary.