Bloomberg: The low-cost movie "Paranormal Activity 3" opened in first place at U.S. and Canadian theaters in the most successful debut ever for a horror film, generating $52.6 million for Viacom Inc. (VIA/B)'s Paramount Pictures. "The Three Musketeers," from Summit Entertainment LLC, took fourth place in its debut with $8.7 million, researcher Hollywood.com Box-Office said today in an e-mailed statement.