POSO, Indonesia — One of three Muslim extremists on trial for beheading three Christian teenagers in Poso, Indonesia, in October 2005 has admitted his role in the attacks. A group of machete-wielding men ambushed Theresia Morangke, 15; Alfita Poliwo, 17; Yarni Sambue, 15; and Noviana Malewa, 15, on the morning of Oct. 29 as they walked to their Christian school.…