The 16-year-old boy was attacked in Elsinore Road, Forest Hill on Sunday afternoon
He was taken to hospital by air ambulance which landed in the grounds of a nearby school. He was said to be in a critical condition.
Reports said the victim was on his way to visit a friend when he was confronted by three men.
A local resident who asked not to be named said: “He was stabbed outside our house and attempted to run off but managed to only get to Cranston Road which is just one road up from me.’’
The suspected are said to be about 19 or 20 and all were dressed in black clothing.
Anyone with information is asked to call the police on 020 8284 8339 or Crimestoppers anonymously on 0800 555111.