WASHINGTON — The Federal Aviation Administration says a third air traffic controller has been fired for napping on the job. An email from the agency says the controller assigned to Boeing Field in Seattle was fired Monday for sleeping on duty on April 11 and Jan. 6. The FAA has been beset recently with a series of incidents involving air controllers. Since March, five incidents of sleeping on the job have surfaced, along with one of an air controller watching a DVD movie while on the overnight shift and an aborted landing of a plane with first lady Michelle Obama and Jill Biden, wife of the vice president, on board. Transportation Secretary Ray LaHood made the rounds of news shows last week to assure travelers that safety is the department’s highest priority.