Authorities say the teens will be charged as adults in the beating in the city of Chester. Police are looking for two more suspects.
Detective James Nolan said Friday the fourth suspect is a 16-year-old girl who was turned in by her father Thursday night. He says the teens appear to have attacked the woman "for fun" this week.
Video shows a group of girls walking along before suddenly deciding to attack the woman sitting on her stoop. They follow the victim into her home as she tries to escape, taking turns beating her.
Police describe the woman as "mentally challenged." She is being treated.