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The Original Star Wars Trilogy - the best by far in my opinion.
Terminator Trilogy - I loved all three of them. The 2nd was the best.
Back to the Future Trilogy - I'll never get tired of watching these. They're modern classics.
Die Hard Trilogy - all great movies. John McClane is the man.
Beverly Hills Cop Trilogy - Great movies! Axel Foley!
Lord of the Rings Trilogy - great movies. Explanation not needed.
Blade Trilogy - entertaining movies although the third could have been better.
Indiana Jones Trilogy - good movies but I was never into it as much as a lot of people
Evil Dead Trilogy - blend of horror and comedy. The first was pure horror, with the second being horror and comedy. The third was mostly just a dark comedy.
Romero's Dead Trilogy - Night of the Living Dead, Dawn of the Dead, Day of the Dead. Great for their time.
American Pie Trilogy - dumb teen movies but very funny
Austin Powers Trilogy - dumb as well but funny as well
Naked Gun Trilogy - how can you not like Leslie Nielsen?
RoboCop Trilogy I was good, 2 was ok, 3 was a joke
Major League Trilogy I loved the first two but the third was unnecessary.
Matrix Trilogy Loved the first one. Reloaded was ok, but Revolutions was just abominable.
Jurassic Park Trilogy The first was great, the second was pretty good as well, although I'm one of the few that believes that. The third was unnecessary. The 4th will just destroy this franchise if they go ahead and make it.
Crocodile Dundee - the first two were enjoyable and the third really wasn't that bad either.
Teenage Mutant Ninja Turtles - grew up with the TV show so I loved the movies as a kid. Looking at them now though, they are bad.