LOS ANGELES — Peter Yarrow, the singer-songwriter best known as one-third of Peter, Paul and Mary, the folk-music trio whose impassioned harmonies transfixed millions as they lifted their voices in favor of civil rights and against war, has died. He was 86. Yarrow, who also co-wrote the group’s most enduring song, “Puff the Magic Dragon,” died Tuesday in New York, publicist Ken Sunshine said. Yarrow had bladder cancer for the past four years. “Our fearless dragon is tired and has entered the last chapter of his magnificent life. The world knows Peter Yarrow the iconic folk activist, but the human