WASHINGTON—The U.S. auto safety regulator said on Tuesday it would require Ford to recall 3 million vehicles with potentially defective Takata air bags, rejecting a bid by the second-largest U.S. automaker to avoid calling them back. The National Highway Traffic Safety Administration (NHTSA) said it was denying petitions filed by Ford and Mazda in which the automakers sought to avoid recalling vehicles with potentially dangerous inflators. The decision also will require Mazda to recall and repair driver air bags in approximately 5,800 vehicles. The recalls will cover various vehicles from the 2006 through 2012 model years. The defect, which leads in rare instances to air bag inflators rupturing and sending potentially deadly metal fragments flying—especially after long-term exposure to high humidity—prompted the largest automotive recall in U.S. history of more than 67 million inflators. Worldwide, about 100 million inflators installed by 19 major automakers have been recalled. Takata inflators have …