The Expendables 3, the latest in Hollywood’s action-star pension plan, confirms the series’ status as the modern action genre’s biggest missed opportunity. Star and story producer Sylvester Stallone has mined his rolodex for a beefed-up (emphasis on the beef) punch-a-thon for him and his bulging buddies and while there are some welcome roster additions, Stallone and new director Patrick Hughes remain unable to dream up material worthy of the star wattage on show. We’re three films deep, and the franchise has yet to deliver a single memorable set-piece. And that’s kind of a problem. At least the films have finally…