“Iron Man 3” is a fine movie as long as you turn off your brain completely. If, however, you view it with even a casually critical eye, you will see a movie full of choices driven by commercial goals rather than what serves the story, characters or even its devoted fan base. Moviegoers who merely want to hear Robert Downey Jr. spout a new batch of Tony Stark quips and see the suit used in new ways will likely enjoy the movie, even though this is the least comical of the three “Iron Man” movies.