Blame it on George Lucas.

In 1983, Hollywood and the moviegoing public came to accept the trilogy – a set of three movies – as the standard, the perfect number of movies in a series.

It was the year of “Return of the Jedi,” not a bad movie in its own right and an appropriate finale to the original films. Of course, it pales in comparison to “The Empire Strikes Back.”

And yes, some movie series (notably “Star Trek”) continued past their third entry.

But thanks to Lucas’ original trilogy and prequel trilogy (sigh), three movies became the norm.

Which meant we’ve seem a lot of good third films (“Return of the King,” “Toy Story 3”) and a lot of bad third films.

Now rumors are sweeping the Internet that director Peter Jackson is considering – perhaps being encouraged – to turn his adaptation of “The Hobbit” from two movies to three movies.

Tolkien’s story is one dear to generations of readers and serves as a great introduction to “The Lord of the Rings.” But it’s hard to imagine how the rather simple story could sustain three movies.

So hear’s hoping Jackson resists the temptation.

But just in case, let’s recall some really awful third movies:

“Superman III,” in which Richard Pryor becomes the Man of Steel’s archnemesis. Interesting how much Christopher Reeve’s costume in that movie foreshadows the muted blue and red colors used in “Superman Returns” and next year’s “Man of Steel.” “Superman III” was topped in its badness only by “Superman IV: The Quest for Peace.”

“Spider-Man 3,” the reason why Sam Raimi is no longer making “Spider-Man” movies starring Tobey McGuire. Does anyone ever want to see Peter Parker dance again?

“X-Men: The Last Stand.” Here’s a great example of how to kill a series. You’d think Sam Raimi and Tobey McGuire were special guests on the set.

“Terminator III.” After the highpoint of “Terminator 2,” a letdown of epic proportions.

And possibly least, “Alien 3.” James Cameron made “Aliens” a thrilling bug hunt. “Alien 3” made us wish Sigourney Weaver had just said no.