BOSTON — A federal judge on Tuesday rejected a request from lawyers for Boston Marathon bombing suspect Dzhokhar Tsarnaev to order prosecutors to turn over evidence about his older brother’s possible participation in a 2011 triple killing. The defense was seeking details of statements from Ibragim Todashev about Tamerlan Tsarnaev’s role in the unsolved killings of three men in Waltham, just west of Boston.