PHILADELPHIA — Three people were killed and 11 others wounded in a mass shooting late Saturday night on South Street amid chaos that erupted on legendary blocks that have long been among the region’s most popular gathering places. “Once it started I didn’t think it was going to stop,” said Joe Smith, 23, who was standing outside the Theater of the Living Arts on South between Third and Fourth streets, when the shots rang out around 11:30 p.m. “It was chaos,” said Eric Walsh who was closing up the outdoor seating area of O’Neals, a bar near Third and South. He saw a young woman collapse to the …