Mmm, just trying to think of some that I love other than Star Wars or Back to the Future...
Evil Dead (Not unpopular, but doesn't get mentioned as a trilogy a lot.)
Spider-Man (I'm counting this because a lot of people hate Spidey 3 and I do not.)
John Carpenter's Unofficial Apocalypse Trilogy (The Thing, Prince of Darkness, In The Mouth of Madness.)
Austin Powers.
...Iron Man?
Evil Dead (Not unpopular, but doesn't get mentioned as a trilogy a lot.)
Spider-Man (I'm counting this because a lot of people hate Spidey 3 and I do not.)
John Carpenter's Unofficial Apocalypse Trilogy (The Thing, Prince of Darkness, In The Mouth of Madness.)
Austin Powers.
...Iron Man?