Maybe Gears of war, the series died after 3 for me, so maybe that counts?
Definetely counts. 4 and 5 are so shallow and empty.
1) Witcher trilogy. I enjoyed every one of them greatly. Never had problems with the flaws of the first and second. The lore, the story, characters, and overall monster hunting always made me enjoy these games to great extents. And 3rd one was cherry on top. Witcher 3 is a game that one can hardly forget
2) God of War PS2-PS3 trilogy. I leave reboot out of it, since it focuses on a different kind of storytelling. I've yet to play Ascension and some other extra games they've released but look forward to them. Just gotta have the damn time! In the end, an important aspect of Kratos' story ended with the 3 so i would say that it can be count as a trilogy of its own since even Santa Monica rebranded their game directly as God of War.
3) AC Desmond trilogy. It told 3 wholly unique characters in Altair, Ezio and Connor. Connor got flak in the first years but latately I see he's seeing the appreciation he deserves. He was a well written character. Revelations and Brotherhood was long games, but served as a continuation of the 2nd game so...
4) Gears Xbox 360 era trilogy. 4 and 5 are so shallow and lifeless that I don't even want to remember them as Gears games. They lack substance, brutality