@Pawel, I think it’s certainly viable to impose external narrative structures on RPGs, because most are flexible enough to accommodate such impositions. I regularly do it in 4-hour PbtA one-shots:
In the first 20 minutes we create characters and setting, in broad strokes.
The next 40 minutes comprise “Act I,” in which the status quo is established through play.
At around the 1-hour mark I offer the PCs a big choice or opportunity improvised from what’s been established (the “first threshold”). In Star Wars: A New Hope, this is when Luke find Aunt Beru and Uncle Owen dead, and chooses to leave Tatooine.
The next hour is the first half of “Act II,” where the PCs deal with the immediate consequences of their choice.
At around the 2-hour mark I try to make sure they cross a “second threshold” – enter a new emotional phase or physical location that raises the stakes. This is when Luke et al sneak onto the Death Star.
The next hour is the second half of “Act II,” where the PCs deal with further and more challenging complications.
At around the 3-hour mark I try to push them across the “third threshold” by hitting them hard where it will really hurt – someone important dies, or betrays them, etc. This is where Obi-Wan gets cut down in front of Luke.
The last hour is the PCs dealing with all of the fallout and last desperate challenges, while I look for ways to tie things together in hopes of putting a bow on it by the end.
This approach was originally inspired by Jon Aegard’s “Dragonslaying on a Timetable” idea. Over several years I’ve gotten pretty good at hitting the marks. The key things for me are not having any preconceived ideas beyond a starting situation (e.g. “You’ve crash-landed on a strange planet”) and asking the players lots of questions so I can improvise all of the important bits out of their contributions. PbtA games are ideal for this tight time frame (my go-tos are Apocalypse World, Dungeon World, Adventures on Dungeon Planet, and Spirit of '77).
I teach in a comics MFA program, and sometimes use games in the classroom to provoke discussion about character motivation, scene setting, and story structure, among other things. Traditional 3-act stories so saturate our culture that we have been trained to expect the pattern (thanks to the George Lucas/Joseph Campbell/Robert McKee feedback loop, 95% of mainstream films hit the requisite beats to the minute), and as a result can sometimes feel like something is lacking if a story doesn’t fit that pattern. (High-five to @shanel for love of David Lynch.)
After we cover the hegemony of the 3-act structure and Hero’s Journey, we talk about alternative approaches, and games are a great way to explore that. As @Nickwedig points out, every game has a built-in narrative structure, defined by its rules. I use Fiasco regularly in the classroom because it creates highly entertaining stories that pivot on a single structural axis (“The Tilt”), few problems are solved, and things generally end in disaster. (Aside: Fiasco is also great for discussing character motivation, when to cut into and out of a scene, how “color” scenes affect pacing, etc.).
I think tabletop roleplaying games offer a wonderful antidote to the hegemony of the 3-act, and should be celebrated for that. While mainstream culture inundates us with formulaic narratives (which of course can be executed well or poorly), RPGs offer a multitude of alternatives. From the “simulationist” pace of old-school D&D campaigns to contemplative nature of The Quiet Year to the physics-driven tension of Dread, these alternatives allow our brains to break out of the ruts imposed on us by the past 40 years of narrative training and enjoy genuinely fresh forms. We are living in a golden age of improvised collaborative storytelling (or maybe “story remembering” to follow @shanel’s idea?), and it’s pretty awesome.