Detectives have named a 14-year-old boy stabbed to death as Kevin Ssali.
The teenager was killed as he got off a bus in Burnt Ash Road, Lee, south London, at about 9.45pm on Saturday.
Three teenagers are in custody in south London police stations after being arrested on suspicion of murder.
Kevin, of Whitworth Road, south Norwood, died in hospital from his wounds. A post-mortem examination is scheduled to take place at Greenwich mortuary on Monday afternoon.
Initial reports suggest the victim was travelling on a route 202 bus through Lee Green towards Charlton.
Both London Ambulance Service and the capital's air ambulance service attended the scene.
A Scotland Yard spokesman said: "As the victim alighted the bus in Burnt Ash Road, he was attacked and stabbed. Officers are trying to establish if there was some sort of altercation on board the bus which led to the incident."
Detective Chief Inspector Mark Dunne, from the Met's homicide and serious crime command, said: "I am appealing for anyone who travelled on the route 202 bus on its way through the Lee area just before 9.45pm on Saturday night to speak to my officers. Did you witness an altercation or argument involving a group of male youths?
"I would also like to speak to anyone who was in Burnt Ash Road, where the victim got off the bus. I need to know what the series of events were which culminated in a 14-year-old boy losing his life."
Anyone with information is asked to contact the incident room on 0208 721 4906 or Crimestoppers anonymously on 0800 555 111.