A 13-year-old Bronx girl was stabbed, pummeled and pepper-sprayed by three other teenagers — one of them the older sister of her classmate, cops said Thursday. The young victim was walking into the courtyard of a building on Ford Street in the Fordham Heights neighborhood around 5:45 p.m. Monday when the three other girls confronted her, police said. “I heard you wanted to fight me,” one of the brutes snarled, according to cops. That girl — who the victim said is the older sister of someone who attends her school — then unleashed pepper spray or Mace at her, according...