A Sydney martial arts instructor has been arrested for the alleged triple murder of a South Korean family living in the city after a child and woman were found dead in a Taekwondo centre, Australian police said. Police arrested the 49-year-old man in hospital late on Tuesday night, Detective Superintendent Daniel Doherty told a press conference. The man, who is under police guard and expected to be charged with three murders, was being treated for apparent stab wounds to his chest, arms and stomach, according to a separate police statement. Police said he arrived at Westmead Hospital in Sydney's northwest…