Police are searching for three people accused of attacking a 19-year-old and robbing him of his backpack in Brooklyn.On Oct. 8, the victim was walking along 56th Street and Sixth Avenue in Sunset Park when three unknown individuals approached him from behind, pushed him to the floor and began to punch and kick him, said police.They then used a knife to cut the straps of the victim’s backpack and fled the scene.The victim was taken to the hospital where he was treated for a cut on the right side of his head.