After a tremendous franchise introduction, the “Iron Man” series fell into a sophomore slump. “Iron Man 2” wasn’t nearly as witty or fun as its predecessor, so it was a big deal when the metal-clad hero regained his luster in the 2012 blockbuster “The Avengers.” Fans should be equally excited about his showing in “Iron Man 3.” Shane Black, previously best known for writing the “Lethal Weapon” franchise, co-wrote the screenplay and took over directorial duties.