LONDON — British police investigating the savage killing of an off-duty soldier in London have arrested three more suspects. Scotland Yard said counter-terrorism officers arrested two men, aged 24 and 28, at a residential address in southeast London. A third man, 21, was arrested separately on a London street at the same time. Police said Saturday they used a stun gun on two of the suspects. All three were detained on suspicion of conspiracy to commit murder. Officers have already detained four others in connection with the murder of 25-year-old soldier Lee Rigby, who was stabbed to death while walking outside the Royal Artillery Barracks in the Woolwich, south London, on Wednesday afternoon.