Two men were stabbed to death and one woman was shot after a weekend of violence in the capital.
Emergency services were called to the scene in Kensal Green, west London, to reports of a stabbing on Sunday afternoon at 2pm.
Despite the efforts of emergency services, he was pronounced dead at the scene.
A 50-year-old man has been arrested on suspicion of murder and efforts are ongoing to trace the victim’s next of kin.
Detective chief inspector Katherine Goodwin, from the Met’s Specialist Crime South team, which is leading the investigation, said: "I believe this terrible incident may have been witnessed by a number of horrified onlookers.
"I urge anyone who has yet to speak to police to contact us immediately. A man has lost his life in violent circumstances and we are doing everything we can to understand what has happened here."
Nearly four hours later, shortly before 6pm, officers were called to the scene in Ramillies Close, Brixton, after receiving reports a man in his 20s had been stabbed.
London Air Ambulance and officers went to the scene in south London but the man was pronounced dead shortly afterwards.
Detectives are trying to trace his next of kin. No arrests have been made at this stage.
And shortly before 9pm, the force received reports that a woman had been shot in Hackney, east London.
Emergency services were called to the scene and found a woman, believed to be in her 30s, suffering from a gunshot wound.
Officers gave the woman chest compressions until the London Ambulance Service arrived. The woman was then taken to a hospital in east London where her condition was deemed to be life-threatening. Her next of kin have been informed and no arrests have been made.
Anyone with information on the incidents should call 101 or call Crimestoppers anonymously on 0800 555 111.