Obviously, the Lord of the Rings Trilogy. Mainly movies for me because I haven't read them. Extended editions, obviously.

But also, I adore the mass effect trilogy. Yeah, the rpg elements get gradually watered down, and the third ones ending isn't the best, but it's still an absolutely amazing Trilogy that I replay yearly. And it all came out in 5 years! Nowadays, single games have 5 years of dev time, at least. In my eyes, it's as perfect as it can be....Once it's been modded a bit.

Half Life.

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...aaaaaaaany day now.

Hitchhiker's Trilogy

The Dollars Trilogy as it’s sometimes called. Italian westerns Fistful of Dollars, A Few Dollars More, The Good The Bad and the Ugly

How to Train Your Dragon Trilogy.

I watched the first one on a ferry, and just hearing the title made me think it was going to be some nonsense. And then it was amazing.

Then they announced a second, and I was thinking what do they expect to do with this and then they gave something intensely heartwarming and heart wrenching. I found it better and deeper than the first.

And then the third. I don't think it was as clean as the other two, but it closed it off so beautifully I was bawling at the end. Absolutely perfect.

as a functional trilogy the Back to the Future film series is pretty high up there.

Avatar the last Airbender had 3 perfect seasons.

The first three Toy Story movies.

There's no way you all didn't cry at the end of #3.

The Phoenix Wright trilogy--the first three original GBA games/DS re-releases. They set up and develop so many arcs that pay off both within each game and across the entire trilogy. I would even go so far as to say that Phoenix Wright 3 is one of the best visual novel games of all time.

Three Flavours Cornetto (wikipedia link)

Assassin's Creed 2, Brotherhood and Revelations aka the Ezio trilogy. I remember playing it when I was quite young and the parkour elements blew my mind. Ezio was a very charismatic character and these games were imo the best Assassin's Creed games, before Ubisoft went to shit and started churning them out every other year.

Half-Life 1,2,2,2

Lord of the rings (Peter Jacksons first run of the series not that shitty hobbit shit) movies extended edition of course. Halo 1-3. Mass effect 1-3.

Lord of the Rings. I have no complaints and am probably one of the few who know why the eagles didn't fly into Mordor.

Gin. Lime. Simple syrup.

I took some artistic liberties with the title.

Actual answer, John Carpenter's trio

The Jason Bourne trilogy.

Half Life, Half Life 2

The Next Generation Deep Space Nine Voyager

I know that technically doesn't count but whatever.

Halo 1-3, both the single player story and the evolution of the multi-player.

Movie: The Lord of the Rings, Back to the Future

Game: HL2 + the two episodic, Metro 1/2/exodus(exodus not my favorite but still great), Halo 1/2/3

Book: Physics/Biology/Chemistry(idk i don't read book)

Mass effect for video games, Rebuild of Evangelion for movies, and Old Man war for Books. I know only mass effect is a trilogy (Andromeda doesn't exist) but I feel like all three stayed with me after finishing them, and the character development and plot lines all had satisfying conclusions.

Spyro 1-3. First game set the mood. Second game refined the formula, last game had fun with it. Still play it to this day.

Whenever I think of the answer to this question, I always lament at how many film trilogies or games could have been absolutely immaculate duologies but were, for various reasons, sort of forced into a third installment through fan expectations, studio pressure, or just plain Hollywood/corporate greed.

It usually begins with a film or a video game that is an unexpected success, something that was written off by the execs that turned out to be not just a work of art, but a pop culture sensation. Star Wars, The Matrix, The Terminator, Assassin's Creed, Mass Effect, etc were probably never intended to have a sequel. Their original plotlines all tied up the loose ends nicely and made for a perfectly adequate self-contained story. Then the second film/game in the series comes out and it's another well received installment. Maybe it's because the second rides a bit on the coattails of the first, or maybe because the first walked so the second could run, it's hard to say, but in every case the second always sets the bar too high. The third installment is typically the one that sours the soup, so to speak. I'd wager that even a really well written story can't really live up to the expectations that fans have for the third installment of a well-beloved series. Having the perfect three-peat is a feat rarely seen accomplished, but nobody ever seems satisfied with just two good pieces of media with no plans for a third.

How to Train Your Dragon.

Books:

Wool/Shift/Dust by Hugh Howey. A well written, immersive post apocalyptic fiction that has a satisfying conclusion.

The Passage/The Twelve/The City of Mirrors by Justin Cronin. Pretty much the same as above.

The Century Trilogy by Ken Follett. A huge read that spans almost a century (from just prior WW1 to the late 20th century), accompanying the same families from several different countries and embedding them into significant world events of the 20th century. Really well written and enjoyable.

If you're in your teens the original mass effect trilogy, from the storytelling to the going through the different genres of games

Star Trek II, III, IV

The Riftwar Saga, by R.E. Feist, starting with Magician. Not LotR levels, but still very pleasing.

Now hold on here...

X-Men Origins: Wolverine
X-Men
X2

It fully explains Wolverine's power set, explains why he has no memory in X-Men, and puts a pin in the storyline with Stryker by X2.

Folks HATE Origins, and I get it, nobody likes that version of Deadpool. That being said, it's NOT the shit show that Last Stand turned out to be, and with this trilogy, you can safely pretend Last Stand never existed.

The love story with Silverfox was sweet and touching.

The battle by battle history sequence with Wolverine and Sabertooth was great.

Even before the transformation, Ryan Reynolds nailed Wade's basic trait as "the Merc with the mouth" before it all goes horribly, horribly wrong.

The only thing it doesn't really explain is how Sabertooth apparently got brain damage between the two.

Matrix 1-3

The 4th one doesn't exist in this house

Explorations into Microtonal Tuning is a trilogy of albums by King Gizzard and the Lizard Wizard that follow on nicely from eachother, all very good albums in their own right, and a third thing I can't think of but would sound rhetorically complete.

Mass Effect and Dark Souls are my favorites when it comes to gaming.

Movie trilogies: Guardians of the Galaxy, Tom Holland Spiderman, Deadpool.

Order the 6 piece mozzerella sticks. You get 2 mozzerella sticks as a trilogy!

Mmmmmmmm, the perfect trilogy!

His Dark Materials trilogy for me. Both the books and the BBC TV adaptation.

croissant, butter, jam.