Two boys aged 14 and 15 have been arrested on suspicion of murdering 15-year-old Tamim Ian Habimana, police said.
They are the second and third boys to be arrested over the stabbing death of Tamim in Woolwich, south-east London, on Monday evening.
The Metropolitan Police said both were arrested on Friday and remain in police custody along with another 15-year-old boy who was arrested on Thursday.
Tamim’s death came hours before the unrelated knife death of another teenager in London.
A 29-year-old man continues to be questioned over the murder of Keane Flynn-Harling, 16, in Lambeth, south London, later on Monday night.
The families of both victims are being supported by specially trained officers.
The number of violent teenage deaths in London this year could be one of the worst in nearly a decade.
As of Tuesday, a total of 21 teenagers had been killed in just over six months this year in the capital, leading to concerns that the grim tally for the whole year will exceed the previous high of 27 in 2017.
This would mean the highest teenage death toll since 2012.