U.S. labor officials are asking a federal court to force Starbucks Corp. to reinstate a group of activists, escalating the legal battle over the company’s response to the union campaign sweeping through its stores.In a filing Friday, the National Labor Relations Board’s Phoenix regional director sought an injunction requiring the coffee chain to bring back three employees who the agency alleged had illegally been fired, forced out, or placed on leave.Workers United, an affiliate of the Service Employees International Union that’s petitioning to represent staff at hundreds of Starbucks cafes, has filed dozens of allegations against the company with the labor board, most of which are still pending. The agency’s prosecutors have found merit in some of those claims, …