A panel of young British readers gave Harry Potter writer J.K. Rowling her third Smarties Book Prize on Wednesday. The prize was awarded for “Harry Potter and the Prisoner of Azkaban.” It was judged best book in the 9-11 age bracket, just as “Harry Potter and the Sorcerer’s Stone” and “Harry Potter and the Chamber of Secrets” were before it. All three titles have topped best-seller lists in Britain and the U.S., despite reservations expressed by some American Christians who object to Harry’s dabbling in the dark arts.