When did movie studios start intentionally making trilogies instead of just standalone films and sequels?

Are they the standard format? Usually movies operate on the "as long as it makes money well make more of it" theory. In which case I'd say 1930s is the starting point at the latest. Hopalong Cassidy, Durango, Billy The Kidd and Three Mesquiters etc all got like 50 entries in their respective movie series

It is hardly a standard format even today. Lord of the Rings was one of the very few projects to go into development as a trilogy from the start, though there have been a couple more that hoped to be a trilogy, but without committing to the production of all three upfront (the recent 28 years later trilogy, for example). The Star Wars prequel and sequel trilogy were intended as trilogies from the beginning, however that's an unusual case where the franchise was already very successful and assocated with the trilogy format; the first trilogy, however, was not originally planned as such. The first film was a standalone film, and it wasn't until after its rampant success that two more sequels were planned; even the gimmick of calling the first one "episode 4" was only added to later re-releases after Empire Strikes Back was made, and starting with 4 was an arbitrary number only intended to make it feel more like a serial. There have been a handful of films that were planned as two films, particularly when adapting a book into two parts, Dune most recently (which only got a third film, based on the second book, after the first two were so successful). The Harry Potter, Hunger Games, and Twilight series all had their last novel split in half, however those were all cases where the franchise was already successful enough that the studio knew that two more films would succeed. So trilogies are hardly a standard movie format. On the contrary, they're much more common in literature, where publishing a trilogy of novels is much cheaper than making three movies (though when book trilogies are adapted, obviously they become movie trilogies if all three end up being made; maybe that's what you're noticing). As for why multi-part stories will often get made as trilogies: it's just the structural appeal of a beginning, middle, and end.

The only real difference is that movies 100 years ago were made quickly and not nearly as expensive, because actors didn't get gigantic salaries - they had to work regularly to make a living, and studios regularly made a shit ton of films. With the rise of wide-screen Cinemascope / Panavision and epic films with casts of thousands, filmmaking got really expensive, too longer to make, and it just wasn't practical to make dozens of films on the same subject. I would say Star Wars / Indy set the tone for a set of three films - if the first is a "blockbuster", you can deliver two more as money-grabs and they're pretty much a sure-thing. So Back-to-the-Future got the trilogy treatment, The Godfather got another chapter to round it out, Jurassic Park and The Matrix got +2 because they were easy money-grabs. Then again, horror films just ignore this dynamic - as long as it makes a buck, you'll get more in a horror series. Also the MCU built a Universe partially on trilogies - but also followed the history of the source material - just add more heroes and villains until people stop buying it. Other attempts to build a multi-film "universe" in modern times have failed. But Godzilla-Kong succeeded.

Star Wars was probably the first example of planning multiple sequels after the success of the first, and George has sad various stuff about wanting the story to have been a trilogy from the start but I don't know how much if it I believe. Back to the Future filmed 2 and 3 back to back in the 80s, but like Star Wars they were only green-lit after the success of the first  Lord of the Rings is the first I can think of that was planned to be a trilogy.

Well Shakespeare did it….

[Three Colors Trilogy](https://en.wikipedia.org/wiki/Three_Colours_trilogy) is one of the earliest examples.

Is it a standard format?

It's rare for a studio to intentionally create a multiple movie series unless they are adapting established source material. Otherwise they are greenlighting multiple movies on original IP without knowing if the 1st one will even turn a profit.

Star Wars was the first, but that was the creators intention, not the studios. Indiana Jones came not long after, not coincidentally by the same creator. So we can actually blame George Lucas for proving that trilogies were popular and profitable, and the fire was technically lit in 1977, but really started to gather steam in the mid 1980's. By the mid-2000's it was pretty common to expect trilogies of big movies (Spiderman, Pirates of the Caribbean) and by the 2010's it was no longer about trilogies but franchises, a never-ending merchandising opportunity, which morphed into the multiverse, to keep the same stars coming back after we've killed them in the name of profit.

Lord of the Rings

$$$

When they found out how much money they could make investing in a franchise

William Powell started in 6 Thin Man movies in the 1930s-1940s.

When they became risk averse. If part one works = built in audience, and helps with cost averaging across the franchise. If not, then it was a one off with trilogy potential.

Naughty Secretaries I through III in the 70's probably showed everyone the possibilities of this format.

Honestly, id say the Star Wars prequel trilogy and the Lord of the Rings (which was in development possibly earlier than Phantom Menace).

The Tobey Maguire Spider-Man trilogy popularized trilogies a lot more, and in my opinion, was one of the last “organic” trilogies

Since parents realized they get peaceful naps with their kids shutting up as long as the movie looks familiar