"All we heard leading up to Halo 3 is that we were going to 'Finish the Fight.' Well, we wouldn't have to finish any fight if the Big M wouldn't have given us the big stiffy with such a shitty ending in Halo 2!"
— Stuttering Craig, ScrewAttack's Top 10 OMGWTF Moments
Sometimes when a movie is made, although no one is expecting that much to come of it, it becomes a surprise hit. Of course, the best way to capitalize on a success is to make a sequel out of it - and as the golden number in Hollywood seems to be three, what better way to hit the jackpot than to make a trilogy out of it?
However, in a lot of these situations the first movie was quite self-contained; after all, if there's no expectation of a sequel, then even if you put in a few just-in-case Sequel Hooks (since producers are nothing if not hopeful) you'll still want to tie the loose ends up enough so that the audience can enjoy the story on its own merits without needing to see a sequel that might never come. If you have a couple of sequels guaranteed no matter what, however, then you can afford to leave the audience hanging in between the second and third movies - after all, they'll be back to see both installments, right?
This can lead to an interesting situation wherein the second and third movies in the trilogy share more direct relation to each other than they do with the original. In essence, what you have is a Two-Part Trilogy - a self-contained first part with heavily intertwined second and third parts. In fact, in some cases the second and third parts of the trilogy might as well be one long movie cut in half and released separately. As a result there are a multitude of recurring pitfalls that can pop up as a result of that mindset; see the Analysis Page for more on that.
Outside of the story, the trilogy might literally be a Two-Part Trilogy - the second and third movies are also often written and produced concurrently (in order to save costs and ideally increase revenue), so where there might be a gap of several years between One and Two, Two and Three might be released within a year (or less) of each other.
In a similar frame of mind, the first game of a video game trilogy will establish a certain gameplay style and the two sequels will be much closer related to each other than the first.
Another cause behind this is that Pop Culture has a very short shelf life, and the executives don't want to waste effort into something that will no longer be a fad in the additional two years it may take to produce the third film.
A Sub-Trope of Movie Multipack, this happens primarily with films and video games.
Spider-Man 2 and 3 fit this. The original definitely stood alone in establishing Spider-Man as a character along with his relationships with Mary Jane and Harry and his battle against the Goblin. The second movie ended with Harry knowing Peter is Spider-Man and discovering his father was the Green Goblin. From there, the third movie starts with Peter being immediately attacked by the New Goblin. Although technically, defeating the New Goblin doesn't seem to be the point of the third film. The Continuity Reboot follows this as well. The Amazing Spider-Man is mostly standalone, while The Amazing Spider-Man 2 ends with a Cliffhanger / Sequel Hook concerning the Sinister Six setting up the next installment.
An interesting comparison between the kinds of trilogy is the Star Wars trilogies; in the original trilogy, A New Hope is clearly made as a movie that can stand by itself (although the narrative is open to the possibilities of later movies), whereas The Empire Strikes Back and Return of the Jedi - greenlit after the success of the first - are more closely linked together and separated by a sequel hook. In the prequel trilogy, however, it was obvious to all that all three movies were going to be made, so all are incorporated together more tightly as a trilogy, but even still, Episode I could be a stand-alone movie, ending on a happy note, while Episodes II and III are more tightly linked through a cliffhanger ending in II (the Clone Wars begin).
The Matrix Trilogy consists of the original film The Matrix, released 1999, followed by the second and third films The Matrix Reloaded and The Matrix Revolutions and the tie-in video game Enter the Matrix, all released 2003 within months of each other. The sequel movies and video game all tell one long story set six months after the first film, about the invasion of Zion; both movies and the cutscenes for the video game were filmed simultaneously, using all the same actors and crew. Features a Required Cliffhanger at the end of Reloaded, with the Nebuchadnezzar being destroyed and both Neo and Bane being in comas.
The Direct-to-Video series of animated shorts The Animatrix, which was made and released in between the first and second films, goes some way towards bridging the gap between the first film and its sequels. Many shorts are prequels to the first film, covering the rise of the machines and the fall of man, while others are set between the first and second films.
The Pirates of the Caribbean trilogy also does this. The first film was made and can be seen as a stand-alone film. Then, after seeing its box office performance, two sequels were scheduled to be filmed back to back. In fact, they had a chance to have some form of resolution or miniature denouement at the end of the second movie, while also preparing the audience for the nature of the third, and building excitement for the coming adventure. Instead, for their Cliff Hanger, they ended with all of the threads dangling. Not to mention that there already was a completely formed resolution for the second movie, Norrington just snatched it away at the last minute.
The sequels also had the weird problem of having too much story AND too little story. For those who are critical of the films, the second film introduced everything and then just messed around whilst waiting for the third to show up and then the third tried to cram too many things in and tie up too many threads.
The Evil Dead trilogy. Though there's a distinct stylistic disconnect between parts 2 and 3, the latter was set up quite explicitly in the former.
2 is sometimes styled as a "remake" of The Evil Dead because Sam Raimi was unable to get reproduction rights for the original. However, it is fully intended as a sequel, and only re-shoots the part it needs to establish the backstory.
Army of Darkness actually retcons the sequel hook from Evil Dead 2 into its exact opposite.
The first Saw movie works pretty well as a stand-alone film, but the next two were pretty obviously made to be tied together. Of course, the third movie was supposed to wrap everything up. The seventh film, Saw 3D, was released in 2010 and finally concluded the series.
Originally, Toxic Avenger Part II and III were one long movie, but got split up, due to being too long.
Intentionally done with The Human Centipede. According to Dread Central, Tom Six explained that, "My goal was that the first film will get audiences used to the concept of a human centipede and prepares them for where everything goes in the next two."
Amusingly inverted in the Dominican Nueba Yol films. There were only two, but the second film was called Nueba Yol 3 because of al superstition regarding second films.
Many people assumed that this would happen with Christopher Nolan's series of Batman films but The Dark Knight Saga has a rare variant where the third movie is more a sequel to the first movie than the second. Batman Begins was (as the title implies) a setup of the protagonist, using a few obscure villains from the comic, and ends on a minor Sequel Hook. The second movie The Dark Knight is a largely stand-alone story about Batman's battle with the Joker, Two-Face and the Gotham Mob. It picks up some loose threads of the first film, pits Batman against The Joker and develop the DA Harvey Dent as a hero. Nolan's intention was to have the sequel go on to feature the Joker on trial, making it more closely linked with TDK, but with Heath Ledger's death they decided on a completely new direction for the third film. Both the first and third movies are about Batman fending off the League of Shadows' attempts to destroy Gotham. The story is brought full circle when it turns out that Bruce's love interest in the third movie is actually the daughter of the first movie's Big Bad. Ultimately, The Dark Knight Rises manages to bridge together a Big Bad connected to Batman Begins while also riding story arcs set up by the ending of The Dark Knight, wrapping up the series as a whole instead of sticking with the story of just one movie.
Inverted in Richard Lester's trio of Musketeer films: The Three Musketeers and The Four Musketeers were produced simultaneously. Over a decade later they were followed by The Return of the Musketeers.
Gettysburg was a stand-alone movie based on Michael Shaara's novel The Killer Angels. It was followed by a sequel based on one of two American Civil War novels written by Shaara's son, with a promise of a third. However, Gods and Generals was not as successful as hoped, and so apparently there is not going to be "The Last Full Measure", making this a two-part trilogy in a different sense.
The first film of the Basket Case trilogy works as a stand-alone, whereas the second two are connected more by tone, cast and plot development.
Both Italian-made Sword and Sandal films from Golan-Globus, The Seven Magnificent Gladiators and Hercules, both with Lou Ferrigno and Sybil Danning, were filmed in the Summer of 1982.
The original Back to the Future was intended to be a standalone film, with its cliffhanger ending intended as a joke. Parts II and III were filmed back-to-back, and were originally written as a single movie titled Paradox which was ultimately split in two due to length. However, the finished films are all fairly equally connected to each other and each have mostly separate main plots.
The Disappearance Of Eleanor Rigby was originally intended to be this with Him and Her, telling the romance from the perspectives of man and woman involved. Later, however, a third film was made, Them, that incorporated the differing perspectives into a single storyline.
Garth Nix's Old Kingdom series. Part one is Sabriel; parts two and three, Lirael and Abhorsen, are actually a single story Divided for Publication, and pick up the story decades later with a new set of protagonists and a completely different Big Bad.
Rendezvous with Rama and sequels is a literary example of a two-part tetralogy. The original book is self-contained (although the last line is a very nice Sequel Hook), while the next three books tell a single story and suffer from severe Sequelitis.
The Indian in the Cupboard. Only the first part was adapted to film.
Matthew Reilly's archaeological adventures, Seven Ancient Wonders, The Six Sacred Stones, and The Five Greatest Warriors.
Dune was originally conceived as one large masterwork, with the two sequels of Dune Messiah and Children of Dune entwined into the story. Considering the original is 412 pages, the second 222, and the third 592, they were obviously split. This creates an interesting case of the first book being easily stand-alone, while the two sequels are more closely connected but can still in a way also be stand-alone. They also allowed for God-Emperor of Dune, basically a midquel that set up the last two books in the series to be written. It's just kinda hard to say where Two-Part Trilogy begins and Trilogy Creep ends, or even what was intended to be a simple, honest trilogy.
Enderís Game began as a stand-alone short story, then was later expanded into Speaker for the Dead. Speaker for the Dead is also sufficiently stand-alone, but the final chapter does have a sequel hook that allows for a sequel if you choose to read it. The sequel also sits surprisingly well as a stand-alone conclusion to Ender's story, but also has a sequel hook if you want to tie up some below-the-surface loose ends. This is where it gets into Two-Part Trilogy country. The final two books in the series, Xenocide and Children of the Mind, are far more connected than the previous books and were originally intended to be a single volume, but were broken off into two with a superficial cliffhanger between them. Children of the Mind returns to being a suitable conclusion, if you count the main character Ender dying, but only opens up the biggest cliffhanger yet.
The Saga of Darren Shan is probably the biggest example on this page, being a twelve-part saga divided into four trilogies which form their own story-arcs. Vampire Blood is about Darren becoming a half-vampire, coming to terms with his transformation and encountering a member of the vampire offshoot, the vampaneze. Vampire Rites is about Darren travelling to the home of the vampire clan and trying to gain acceptance by them, while uncovering a conspiricy to destroy the clan. Vampire War is the hunt for the Vampaneze Lord, whose death can end the war between the two clans. Vampire Destiny is about Darren learning disturbing revelations about the implications of the war, while it comes to its conclusion. Strangely, Vampire War and Vampire Destiny form their own Two-Part Trilogy, as the last two books of each form a complete storyline in contrast to the first, which are more establishing the change in the story after the Time Skip from the halfway point (in the case of War) and briding the Grand Finale with the WHAM Episode that ended War (in the case of Destiny).
The Warrior Cats series by Erin Hunter works this way, except with series each containing six books. The first series works well by itself and nicely wraps up the ending. The ending of the second series has a few unsolved puzzles and sequel hooks, but can also work fine as the final ending of the whole book series. However when the third series ended, many plot points were still unresolved and it was clear it was just setting up the stage for the fourth series.
The Dark Is Rising is a two-part pentology. Over Sea, Under Stone is a stand-alone, fairly standard kids' treasure hunt, with very little magic, a self-contained story, the treasure found and the bad guy defeated. The next four books introduce new characters (including Will, who takes over as the main character), magic, a bigger bad, an epic background war, mythological tie-ins, and a story that all links together.
The Twilight series is an odd example. It originally was supposed to be a two-part series, but the second book, Forever Dawn, was broken into New Moon, Eclipse, and Breaking Dawn. This resulted in development of the wolves and Jacob, with less emphasis on Bella's point of view of her pregnancy.
Interview with the Vampire is basically a stand-alone memoir of Louis's life in the nineteenth century with Daniel's search for Lestat making for an ambiguous Sequel Hook. Starting with The Vampire Lestat, Louis and Daniel are pushed aside in favor of a multi-part storyline detailing Lestat's plot to awaken Queen Akasha, the resulting disaster, and his search for redemption.
The Land of Elyon series is two examples in one. The first book can stand on its own, and the first three can stand on their own, but not the first two or first four, making the first three a two-part trilogy and the whole series a three-part pentology.
The Millennium Trilogy also follows this format. The plot of book 1 is "find out what happened to Harriet Vanger" while book 2 and 3 are concerned with "find out what happened to Lisbet Salander and punish those responsible".
The first book of the Haruhi Suzumiya series was written as a standalone story without much of a Sequel Hook, but it became so popular that ten sequels (and counting) were produced. The sequels are literally intertwined: most contain several different story arcs occuring some random amount of time after the events of the first book, which has become more like a giant prologue and character introduction than an actual installment. The fourth book lampshades its own giant prologue, probably partly as a reference to the first book's transformation.
Peter and the Starcatchers has the first book in the trilogy be roughly standalone with all of the major plot events more or less resolved; when Cerebus Syndromereally kicks in around Shadow Thieves, the books start to directly continue into one another. Sword of Mercy is more or less the same. However, if one looks by the major story arcs in the series, it actually does form a trilogy, with Starcatchers and Sword of Mercy being relatively standalone. (Relatively because Sword of Mercy still continues an arc from Secret of Rundoon and Shadow Thieves)
The Wheel of Time series is a rather special case, as it was originally intended by the author to be a Two-Part Trilogy, with the first book being capable of standing alone should it not be sufficiently popular... as it turns out, there was so much content to be put into the other "part" that it is now a Two-Part Tetradecology. Indeed, it was such an extensive story, it suffered from Author Existence Failure when Robert Jordan died, and Brandon Sanderson had to be brought in to finish writing what was meant to be the twelfth and final book... but which ended up being turned into three books, due to the sheer volume of content still to be written.
The first novel in The Hunger Games trilogy wraps up after the conclusion of, well, The Hunger Games. The second two books deal with the fallout of the first book and the revolution, with the last line of the second book, Catching Fire, being a Wham Line.
A lot of readers feel the Divergent trilogy is one of these; the difference being that it's the third book, Allegiant, that seems to feel out of sync with the other two, to (for many readers) its discreditóalternating between two narrators where the first two books were told entirely in Tris's voicenote yet with Tobias sounding just like her just so the narrative can continue after her death, and sacrificing the narrative energy of the first two books for a lot of exposition which shouldn't really be necessary at that point. A piece in Buzzfeed when the first movie adaptation came out called it "listless, anti-climactic, and you really get the sense that Roth didn't know where she was going with the story," noting that the four top-rated reviews of the book on Amazon share this assessment. It's been suggested that Roth's publisher got her to lengthen the first book into enough for two; the producers of the films have announced they're splitting Allegiant into two, suggesting they see the same problem.
Mistborn started like this; Brandon Sanderson figured that he had hitherto only published one novel (Elantris), so while he had more or less planned out the entire trilogy to an extent in his head, he intentionally wrote the first book as a standalone, though he attached a Sequel Hook to the ending namely, the Lord Ruler warning that, while he was the Big Bad, something far worse than him was out there and only he had been stopping it. The second book picks up threads from the first, but ends on a Cliff Hanger that sets up the third and the villain of book three was The Chessmaster behind the events of book 2 (and responsible for Lord Ruler's backstory that lead to book 1).
A downplayed example in His Dark Materials. The Golden Compass ends as a very well-contained novel. However, the ending of The Subtle Knife is a cliffhanger that simply demands one scramble to find the nearest copy of The Amber Spyglass.
Played straight with the first trilogy of The Riftwar Cycle: the first book, Magician, stands alone while the second two, Silverthorn and A Darkness at Sethanon, tell a two-part story. Inverted with one of the later trilogies, Conclave of Shadows; the first two books, Talon of Silver Hawk and King of Foxes, tell a two-part story, while the third, Exile's Return, features a Perspective Flip, change of setting and a separate story of redemption for the earlier parts' villain, as well as setting up a later series.
The Kane Chronicles by Rick Riordan are also an example of this. The first book ends with Carter and Sadie defeating Set and essentially neutralizing him as a threat. Then it turns out that Set's dragon was actually possessed by Apophis, the god of chaos. Apophis then directly becomes the antagonist for books 2 and 3.
The Last Dragon Chronicles is an inverse of the way it usually happens - the first two books form a more complete and coherent story, with the third one being more seperate.
Inverted with Nick Perumovís (not it wasnít that kind of inversion) Ring of Darkness, set in Tolkienís Middle-earth. The first two installments (Elven Blade and Black Lance) can be considered as a closed duology and were originally written by author in 1985-1991 as a form of fan fiction for his friends. The third book (The Adamant of Henna), however, was written after the first two were published in 1993, turned out to be bestsellers and spurred Perumov's popularity. It is set about ten years after the events of previous books, tells about a new peril rising and basically serves as a gateway to the authorís own setting.
City of Fallen Angels was originally supposed to be a one-shot follow-up to The Mortal Instruments trilogy, until Cassandra Claire decided to make it the start of a second trilogy. It shows.
A variation in Diane Duane's Rihannsu series, which is more of a three-part quintology. The first two books are interconnected but stand on their own pretty well, but the next three, written a dozen years later after the Star Trek copyright holders lifted their restrictions on recurring original characters, are a tight trilogy. For some bizarre reason the omnibus edition contains the first four books, omitting The Empty Chair.
The Diogenes Trilogy in the Agent Pendergast series contains Brimstone which is mostly self-contained and the more directly-related Dance of Death and Book of the Dead. However, Brimstone winds up subverting this in the end as it ends in a clear cliffhanger that leads into Dance of Death.
The final three episodes of Series 3 of Doctor Who ("Utopia", "The Sound of Drums" and "The Last of the Time Lords") tell a linked three-part story about the Doctor's battle with the Master, and they have been officially listed as a trilogy by the BBC in many official sources. However, "Utopia" is a largely standalone story about the Doctor and Jack Harkness attempting to save a beleaguered human colony in the distant future, and the Master's involvement isn't made clear until the very end; "The Sound of Drums" and "The Last of the Time Lords" are much more obviously interconnected, as both are about the Doctor and his allies leading a resistance against the Master in his "Harold Saxon" guise in the present.
Frank Zappa's Hot Rats got two sequels released close to each other, Waka / Jawaka and The Grand Wazoo. Then, several years after that, there was the unoffically named Lšther(German for leather) trilogy, consisting of Studio Tan, Sleep Dirt (sometimes referred to as Hot Rats III), and Orchestral Favorites, making up a three-part sexology. Confused yet?
An Older Than Steam example occurs in William Shakespeare's two historical tetralogies, which follow this pattern to a certain degree (they are both, effectively, three-part tetralogies). Henry VI Part 1 focuses primarily on the wars in France, while the arc of parts II and III, and Richard III, is about the Wars of the Roses. Similarly, Richard II focuses on the decline and fall of the eponymous king, while Henry IV, Parts I and II feature the rise of Prince Hal, the future Henry V, culminating in his defeat of the French in (surprisingly) Henry V. While the trend in performance is to present the plays as one big cycle, scholars often dispute the degree to which the whole thing was planned out.
The series as a whole shuffles its settings by shifting in time periods, settings and featuring new Player Character between series shifts. Going from The Crusades and Altair to The Renaissance and Ezio between I and II. However the Ezio trilogy plays it straight. Assassin's Creed II and Assassin's Creed: Brotherhood chronicles the childhood, coming-of-age and rise to power of the Italian Brotherhood as Ezio Auditore goes from Novice to Mentor with virtually no time gap between them. However, Assassin's Creed: Revelations takes a Time Skip, goes outside Italy to Istanbul, Ottoman Turkey and features a completely different supporting cast, and wraps up the story of Altair from the first game.
This is exactly the case with the Halo series. Halo 2 ended on an obvious cliffhanger, leaving many, many fans feeling cheated until the success of Halo 3 quelled many of those sentiments. Halo: Combat Evolved itself, however, is an aversion; where most first entries in a trilogy were made to stand alone, the first Halo already established itself as just a small chapter in an ongoing conflict, with the last line even being "No...I think we're just getting started", plus an outright Sequel Hook (343 Guilty Spark revealing himself to be alive) after the credits. Combined with how extensive the Expanded Universe was even from the start, it's safe to say that this series was always meant to consist of multiple parts, even if its originating company is no longer running the show.
Unlike Halo, Knights of the Old Republic has suffered the opposite. The ending of the first game is quite clear, the player has either saved the day, defeated the Big Bad, destroyed the Death Star knock off and is universally loved, or has kept the Death Star knockoff for themselves and unleashes their new fleet on the galaxy to take over. At the end of the sequel though, good or evil ending, it just ends with the character's ship flying off into space, presumably to go and find the True Sith. That was in 2004, and the plot threads were only recently followed up on with the release of a sequel/spinoff MMORPG, Star Wars: The Old Republic. In relation to the classic example of the original trilogy, Star Wars: The Old Republic does this as well despite being only one game: each class has a personal story questline which is divided into three acts or chapters; the first chapter ends with the character triumphantly achieving their goal so far, while the second chapter tends to end inconclusively with a setback and a direct lead into the third and final chapter.
The first three Phoenix Wright: Ace Attorney games fit this trope, though with an odd twist— 1 and 3 have more connections to each other than 2 has to either of them, though this came about in much the same manner as any other Two-Part Trilogy. And of course, the fourth game was going to be completely unconnected until the Executive Meddling.
Save for some minor plot threads and a semi-cliffhanger, Kingdom Hearts I stood on its own more than its two immediate sequels Chain of Memories and Kingdom Hearts II, which both opened up a new plotline involving Sora, Donald, and Goofy facing Organization XIII while searching for Riku and King Mickey. The English or Japan-exclusive remakes often add extra content to set up sequels / spinoffs. The first game's Final Mix added Sora's first encounter with an Organization member, who appears in Hollow Bastion to drop all kinds of cryptic foreshadowing before and after a Sephiroth-level boss fight, and only through playing Kingdom Hearts II do you learn that this member was Xemnas, the LEADER.
The Modern Warfare trilogy. Call of Duty 4: Modern Warfare tells a largely standalone story about the Russian Civil War, and about Task Force 161's battle against the Middle Eastern terrorist Khaled Al-Asad and the Russian Ultrationalist leader Imran Zakhaev. Modern Warfare 2 and Modern Warfare 3 tell a much more obviously connected two-part story about a war between Russia and the United States, with Vladimir Makarov as the overarching Big Bad connecting both installments. Even the titles emphasize this: the series dropped "Call of Duty" from its name in the second installment, once it became clear that the "Modern Warfare" brand was successful enough to carry its own series.
All signs also pointed to this happening to the God of War series. The ending of the first game implied that Kratos would remain the God of War until the present day. The second game started with Kratos losing his godly powers, and ended without them restored. It also ended on an obvious sequel hook of Kratos leading the Titans to war with the Olympians. It was more apparent in the third game, but they also managed to explain away some of the differences between the first and second game, such as why Kratos can still kill gods without Pandora's Box, and why the gods are more malicious in the second game.
Xenosaga actually flips this around; in this series its the first and second entries that are closely interconnected together (as episode 1 was originally meant to be longer, but was then split in half) and the 3rd entry that stands more on its own, having started off with a time skip. Arc The Lad follows a similar pattern; Arc 1 and 2 are basically one long video game, while Arc 3 skips ahead many years and stands on its own.
Phantasy Star Universe has this problem in the main story episodes. The first episode is a self-contained story about the SEED invasion, which you eventually defeat once and for all. Then Episode 2 introduces the Illuminus, who turn out to be behind a lot of the SEED trouble, and they essentially bring about another SEED invasion. Episode 2 ends on a cliffhanger with the GUARDIANS colony being destroyed and the Illuminus more or less victorious. Episode 3 finally has you tie up the loose ends of the previous episode, and end the threat once and for all... again.
The Dragon Age series has been accused of this. It was planned to be a series from the start, but the first game is fairly self-contained with an ending that grants a fair amount of closure for most of the characters (with only one major sequel hook, and only if you did Morrigan's ritual). Dragon Age II, on the other hand, ends on a cliffhanger that pretty much forces you to buy the upcoming expansions and sequels. The third game involves a setting-wide Darkest Hour - while the series may not end there, they are following this pattern.
Zig-Zagged and justified with Fire Emblem Akaneia. The only real "trilogy" in the series; but it wasn't even intended as one - What's referred to as "Fire Emblem 2" is in fact Fire Emblem Gaiden, a Gaiden Game.
The sixth, seventh and eighth seasons of Red vs. Blue make up the Recollections Saga, named in order: Reconstruction, Recreation and Revelation. Reconstruction leaves a few plot points open ended, such as the fate of Church, Wash and the Meta, as well as a cameo by Tucker acting as a Sequel Hook, but for most part, it was a stand alone piece. Recreation however, ends on a dual cliffhangar of Washington shooting Donut and Lopez, while Sarge, Grif, Caboose, Tucker and Epsilon have to contend with a squadron of aliens.
The Stick figure series Shock is an inversion of this trope. The first two are a two-part episode and the third one basically stands on its own.
The Fairly Oddparents "Wishology" trilogy is intentionally like this, despite it being known from the beginning it would be a trilogy. Part I is a stand alone story involving Timmy's first battle against The Darkness. Part II ends on a cliffhanger, which is resolved in Part III.
Happened with the sequels to Arthur and the Invisibles, to the point that the U.K. distributor edited the two films (Arthur and the Revenge of Maltazard and Arthur and the War of the Two Worlds) into one (Arthur and the Great Adventure) and the U.S. DVD release was a simple two-disc set of both films instead of separate releases.
The Legend of Korra has a mostly self-contained plot in Book 1, which was originally going to be the entirety of the series, but Books 2, 3 and 4 form a two-part trilogy, with the events of Book 2 being more standalone but creating the situation for the more inter-linked Books 3 and 4, which focus on the conflict against the Red Lotus Society in a changing world.
There are plenty of aversions, but these works were made and designed either as one entire story, or all as individual stories.
The Lord of the Rings film trilogy. None of the three films really stand alone at all; they were all approached with the assurance that all three would be successful, and all were filmed within the same time period. In other words, all three films had the characteristics of the "second part" of a Two-Part Trilogy. It was a big gamble, especially for such expensive films, which of course is why this trope happens more often than not. It helps that the movies were based on a single doorstopper novel split into three volumes at publishing time; the trilogy was pre-existent, so it was easier to tailor the movies around a three-part structure. Tolkien was not happy that the publishers made him divide it for publication. In adapting the story to film, both part one and part two end in different spots in the story than their respective books, as the films were designed from the beginning as a story in three parts and needed more appropriate spots to end the first two parts. The Fellowship of the Ring ends only about a chapter later than the book (Boromir's death is the first chapter of the second book). The Two Towers has a bigger gap in the endings, for a combination of two reasons. First, rather than split the tale into Frodo and Sam/Everyone Else portions like the book, the movies show everything chronologically, and the ending of the second book (Shelob and Frodo's capture) doesn't happen until the siege of Minas Tirith has begun. Second, after Helm's Deep, anything else was going to be an anticlimax. Instead, Frodo and Sam spend an extended amount of time with Faramir.
The Transformers sequels deliberately subverted this trope. Director Michael Bay has gone on record saying that he hates this form of movie-making and doesn't want to hold back on the current movie because he wants to save something for the sequel, "Let's go for broke on this one." Each movie aims to tell a complete story, although it has plenty of hints towards future movies.
The Last Airbender was written from the beginning to be three films matching the three seasons of the show, only filmed individually. Key plot points for future movies such as Ba Sing Se, Sozin's Comet, and Azula are emphasized without ever being even shown, let alone paid off. Then, of course, the film was a critical and commercial disaster upon release and no future movies will be made. This is why so many studios use this trope — a bad bet leaves your already bad movie looking even worse.
The "Spock trilogy" of Star Trek II: The Wrath of Khan, Star Trek III: The Search for Spock and Star Trek IV: The Voyage Home. Wrath of Khan was originally supposed to be the final Star Trek film involving the original cast due to how tepid the reaction was towards Star Trek: The Motion Picture, hence why the screenwriters chose to kill off Spock. Wrath Of Khan turned out to be a success that revived interest in more films and, even more surprisingly, Leonard Nimoy enjoyed the movie so much he was interested in returning. The entire plot of The Search for Spock revolved around bringing him back and was resolved, leaving some dangling plot threads surrounding the serious laws that were broken to make it possible. The Voyage Home concludes those plot threads, but the primary story of the film was something unrelated to Spock or their legal problems (except in as much as it gave them a chance to save the world and thus return home heroes instead of just criminals). Overall, the "trilogy" was more accidental than anything else.
The Complete World Knowledge trilogy. Only the first two volumes are out so far, but it looks like all three will share an equal degree of cohesion.
The May Bird book series is more like an inversion; while most examples here are of a trilogy composed of two story arcs with one in the first installment, and the other split between the second and third, here the first arc is covered by Books 1 and 2 with the second arc covered in Book 3.
According to one account, J. R. R. Tolkien wrote The Lord of the Rings as a single long novel. However, at the time of publication in the early 1950s, paper was still under Britain's post-war rationing and not enough was available for his publisher to put out a large enough first printing. So, they suggested to Tolkien that he divide it into a trilogy, which he did.
Mass Effect is actually an aversion of this trope as Bioware had ALWAYS planned on it being a trilogy and essentially planned the plot out from the beginning. They aren't just trying to milk everything. The series has seen natural progression so far with the first two games, with each story standing alone and while the previous games are referenced they merely form a backstory and not key plot points to understand the current story. The second gameallows you to import your savegame from the end of the original Mass Effect, and the loading screen tips repeatedly advise the player to keep their old savegames around for Mass Effect 3. The games are all very immersive RPG's and so you aren't just saving your character's appearance, but the choices you made in the first game influence your experience of the second. In particular, a key decision made at the end of the first game reflects the way certain characters treat your character in the second game, as a Savior or an Anti-Hero. However, in terms of gameplay and general look-and-feel, the latter two games are much more similar to each other than either is to the first game, although that can be chalked up to Early Installment Weirdness.
The Gears of War series was developed with each installment standing on its own. You are introduced to the basic premise via a voice-over (which is basically a long-standing war between two factions) and then the characters show up and introduce themselves. The first game is about a critical mission that puts you directly in the middle of the conflict with a Sequel Hook at the end. The second game builds upon things that were brought up in the first game but otherwise tells it's own self-contained story. The third game wanted to avoid locking people out of the story so it will also be self-contained, focusing on telling a story rather than simply resolving questions.
Final Fantasy XIII was fairly self-contained, but left a few unanswered questions related to the Fal'cie and their creators. The sequel retcons the ending of the first game, but is also rather self-contained, focusing less on the Fal'cie and more on time-traveling shenanigans, and Caius Ballad's plan to destroy everything. The second sequel is a direct result of things that happened in the second game, but also gets around to answering some of the questions that have been around since the first game.
The developers of Halo 4 have made it clear from the start that it was always meant to be the beginning of a second trilogy.