Blame George Lucas or Peter Jackson and their blockbuster trilogies, but three has arbitrarily become the movie industry’s magic number when it comes to the narrative arc of franchises.The reasons are logistical — once “Avatar” or “The Matrix” becomes a smash, there are savings, or at least efficiencies of scale, in doing two sequels at once — and practical, since actors and directors understandably don’t want to spend entire careers tethered to a single character or concept. In hindsight, the genius of the “Harry Potter” franchise was catching them young. Whatever the cause, the result is a trio of Batman movies from Christopher Nolan, three Spider-Man films by Sam Raimi, two “Star Wars” trilogies, “The Lord of the Rings,” even three “The Bourne Identity” adventures. And now “The Hobbit,” initially designed as two movies, is expanding into three as well. Had James Bond made his cinematic debut in the current environment, the super-spy would have killed Blofeld, told M to bugger off and decamped for the Bahamas at the close of “Goldfinger.” Although the trilogy has become the de facto standard, building toward a third movie weighted with the promise of finality creates some inherent complications. For starters, studios have no intention of letting wildly popular franchises simply ride into the sunset. So with the property almost sure to be revived in some form, providing elements of closure within a trilogy is often problematic. Watching “The Dark Knight Rises,” there was a nagging sense, for all of director/co-writer Nolan’s meticulous plotting, that Warner Bros. will never allow Batman to rest very long — just as Bond has stayed in her Majesty’s service, in real-world terms, long enough to become “The Spy Who Hit Me With His Walker.” Critic Roger Ebert cleverly addressed the matter of movies as products with a finite shelf life in a 2011 Newsweek essay on the subject of sequels. “Once a brand as been established in the marketplace, it makes sound business sense to repeat the formula,” he somewhat wearily conceded. “When Procter & Gamble discovered that Ivory soap would float, do you think they came out two years later with a soap named Buoyant?” Granted, some filmmakers have gone to great lengths to create organic spinoffs that can justify further adventures, whether that involves prequels (as with “Star Wars” and now “The Hobbit”), breaking out individual characters or, in the case of something like “X-Men,” a bit of both. On the flip side, Sony hit the reset button on “Spider-Man” in near-record time, retelling the character’s origin and finding new stars as an excuse to unspool this summer’s release a mere five years after the first trilogy wrapped up. Universal, meanwhile, expanded the “Bourne” universe to introduce a wider conspiracy and new super-soldier, yielding what’s in essence a title, “The Bourne Legacy,” in search of an actual movie to go with it. It’s perhaps an improvement over “Bewitched” syndrome — substituting in a new Darrin without explanation — but as constructed only fuels cynicism about studios churning out widgets in lieu of creative inspiration. While it’s a given that popular titles will kept alive by any means necessary, the diminishing artistic returns seem to reinforce a point by Wall Street Journal critic Joe Morgenstern in his recent pan of the new “Total Recall.” “Remakes aren’t supposed to top originals, or even to match them,” he wrote. “They’re meant to draw new revenues from new audiences.” Even so, one can argue with filmgoers becoming more sophisticated about the business and its avaricious nature. There’s enhanced incentive and self-interest in improving the sequel process — starting with a savvier approach to the elaborate fiction that the “final chapter” of a successful trilogy represents anything more than a forced respite before inevitably beginning anew. Although three-and-out works as a baseball metaphor, basketball offers a more appropriate analogy, given the uneven application of the rule of three to studio franchises, creating headaches for those responsible for keeping money and merchandising flowing. Simply put, if the movie business is going to hit a higher percentage with its long-range game of extending franchises, studios are going to need some three-pointers.