BOSTON — Three college friends of Boston Marathon bombing suspect Dzhokhar Tsarnaev were arrested and accused Wednesday of removing a backpack containing hollowed-out fireworks from Tsarnaev's dorm room three days after the attack to keep him from getting into trouble. In court papers, the FBI said one of them threw the backpack in the garbage — it was later found in a landfill by law enforcement officers — after the young men concluded from news reports that Tsarnaev was one of the bombers. Azamat Tazhayakov and Dias Kadyrbayev, both 19-year-olds from Kazakhstan, were charged with conspiring to obstruct justice by concealing and destroying evidence.