Threequels are an interesting part of cinema, everyone can talk about the best and worst sequels, but third instalments are a little different. Firstly, not that many of them are made and those that are, are usually saved for only the most lucrative of franchises and superhero movies. Secondly, naming a good threequel is a very hard task.
Outside of mega franchises such as Harry Potter and The Lord Of The Rings, very few good threequels have been made. The Godfather Part III couldn't live up to its predecessors, Back to the Future Part III is intrinsically flawed and Return Of The Jedi is the consensus weak point in the original Star Wars trilogy. But films like Toy Story 3 and Indiana Jones And The Last Crusade keep that fire burning for good threequels.
Threequels are so hard to get right because not only do you have to top the first film, you have a second film to better as well. By the time it comes to a third part, ideas for plots and character development are bound to be waning, especially if there is no prior source material to rely on. Nobody wants to see the same film a third time, so pressure is on the makers to deliver an exciting entry into a franchise, only it often fails and you end up with Transformers: Dark Of The Moon instead.
With threequels however, studios see the potential for huge box-office returns and the bottom line is often the reason threequels get made. Nobody decided to finance Blade: Trinity on the basis that it was going to revolutionise cinema after all. However, they often let down long-term fans of a particular franchise, most notably Spider-Man 3 crushed a million fanboys' dreams, despite it doing astronomical business commercially. Speaking of which.