Forget about Darth Vader. Luke Skywalker’s newest threat is the Avengers.

One week before it hits theaters, “Avengers: Infinity War” is poised for a massive box office debut that could unseat 2015’s “Star Wars: The Force Awakens” as the biggest opening of all time in the U.S. and Canada, currently $248 million.

The third “Avengers” movie has...