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.

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

Hitchhiker's Trilogy

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.

Three Flavours Cornetto (wikipedia link)

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.

The Star Wars trilogy.

Half-Life 1,2,2,2

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.

The Mistborn trilogy by Brandon Sanderson. The original one that is, the second trilogy in the same universe not as much.

A good example for a writer who managed to place two consecutive trilogies in the same universe is Trudi Canavan with the Black Magician & Traitor Spy trilogies.

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.

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.

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 increasingly inaccurately named Hitchiker's Trilogy by Douglas Adams.

John Carpenter's Apocalypse Trilogy (Prince of Darkness, The Thing, In The Mouth of Madness)

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)

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

The mass effect trilogy. Yeah, shut up, the ending is great. NEVER had a problem with it. It's a videogame, really expecting that the ending will take into account all the decisions over 100 hours of gameplay and dialogue and give you a very personal ending for you is lunacy. Even real life doesn't work like that. The 3 endings with slight variations depending of your war assets was more than enough for me.

The Next Generation Deep Space Nine Voyager

I know that technically doesn't count but whatever.

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.

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.

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

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.

Star Trek II, III, IV

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.

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

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

Mmmmmmmm, the perfect trilogy!

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.

I’m re-reading the Thrawn trilogy by Timothy Zhan and it’s a lot better than I remember it.

Last time I read it, I was 10. I’m now 40 and it’s a much more interesting story now.

Was inspired when I watched Andor and it reminded me of Zhan’s take on the SW universe.

How to Train Your Dragon.

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.