How do you justify separating Order of the Phoenix from the other Yates movies? Why not be content with the David Yates Harry Potter Tetralogy Box Set?
I yield to your superior knowledge of the Rocky franchise. I also agree that there is no narrative imperative for the existence of an additional Toy Story movie and I would rather they leave the series be, not to mention how much of its time Pixar is dedicating to sequels nowadays.
I went back to 1980 in BoxOffice Mojo to look at the highest grossing movies that were part of trilogies and here are the series I would consider, in chronological order:
Star Wars (3 trilogies)
Indiana Jones (of which only three movies were ever made)
Rocky I-III (as per your case)
The Mel Gibson Mad Max Trilogy
Back to the Future
The Godfather Trilogy (did not know they had made that much money)
The Original Robocop Trilogy
The Teenage Mutant Ninja Turtles Original Trilogy
The Dumb and Dumber Trilogy (I never said these had to be good)
Men in Black
The Mummy Brendan Fraser Trilogy
The Original X-Men Trilogy
Lord of the Rings
Bridget Jones Trilogy
Sam Raimi's Spider-Man Trilogy
The Original Matt Damon Bourne Movies
Gore Verbinsky's Pirates of the Caribbean Trilogy
The Dark Knight
Rebooted Timeline X-Men Trilogy
The Santa Clause
The Apes Trilogy
The Wolverine Trilogy (I think these work as standalones but I am not going to spend any amount of time trying to remember the first two.)
Night at the Museum
You can disagree about where to draw the line for blockbusters. Adding more series would require me possessing the kind of knowledge 1SO has of Rocky about all the other series. Also, these are the domestic grosses, I may be ignoring some Gozdilla trilogy that was massively popular in Japan or something.
Other things that occurred to me:
Only the first Aladdin is widely known, but there is technically a trilogy of those.
How do you justify including the "Man With No Name Trilogy" (an MGM marketing ploy) on your list?
Totally forgot about replying to this. I just checked their box office returns and I honestly thought they had had more mass appeal than that. I will have to amend the list and translate the trilogy into the auteur trilogy list.