The original Star Wars trilogy is comprised of the last 3 movies in the chronological story.
But yes, they are also the first three movies to be filmed.
This is what makes it weird in conversation to refer to liking "Star Wars" when asked what your favorite movie is. Subtitles help- so that you can say "Episode IV" or "A New Hope"- but honestly, no one in 1977 really referred to the movie as anything other than "Star Wars" from what I can tell. "The Empire Strikes Back" is technically "Star Wars: The Empire Strikes Back" but that's too many syllables for people to say, thus it's just referred to as "The Empire Strikes Back" and the same deal for "Return of the Jedi"- that all works fine until you get to "The Phantom Menance", "Attack of the Clones" and "Revenge of the Sith".
Two different generations have separate series of movies in mind when thinking of "Star Wars", it seems. I accept all 6 as part of the cohesive storyline of course. And I can see how it all fits together.
Of course, then you have to step even further back to incorporate it all into the cohesive Star Wars expanded universe that includes the books and comic books and video games, TV-movies and specials, cartoons and of course- random guest star appearances.
So it really should be- "Star Wars: A New Hope", "The Star Wars Holiday Special", "Star Wars: The Empire Strikes Back", "Star Wars: Shadows of the Empire", "Star Wars: Return of the Jedi", "Star Wars: The Truce at Bakura", etc.
(I can't remember where exactly the "Star Wars: Droids" and "Star Wars: Ewoks" cartoons fit in or the "Star Wars: The Ewok Adventure" and "Star Wars: Carvan of Courage" fit in either for that matter... probably after Episode VI, I would think)
But then you also have to fit in the Star Wars appearances on "The Muppet Show" and "Sesame Street"