On Monday, officers were called to the stabbing on Leytonstone Road, Stratford, at around 4.40pm, where a victim - in his 20s - was taken to hospital.
The Met is treating it as a murder investigation and appealing for witnesses.
A spokesman for the police said: "At the scene a 15-year-old boy suffering from a gunshot wound was treated by London ambulance service (LAS) before being taken to a central London hospital for treatment".
No arrests have been made.
"A 12-year-old boy was treated at the scene before being taken to a west London hospital where he remains now", it said in a statement.
Detectives from the Homicide and Major Crime Command are investigating.
The 13-year-old - who had been walking along the road with his parents - was also hit by a shotgun pellet to his head, he has since been discharged from hospital.
The manager of a betting shop nearby spoke of one of the boys being "proper covered in blood" but said he was "sitting calmly as the paramedics were looking after him".
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She added that they are investigating whether the incidents, at two locations in close proximity in High Street on Sunday, were linked.
"As we have already stated, over the Bank Holiday weekend additional officers are on the streets across the capital, working hard to keep London safe".
"I want the community to help us tackle this problem". His condition is not life-threatening.
In the latest incident, a 43-year-old man was stabbed in Ealing after a dispute over a man's driving on a residential street.
Rhyhiem Ainsworth Barton was gunned down in London less than 24 hours before two other teenage boys - one aged just 13 - were shot in broad daylight in one of the capital's streets. His injuries are not believed to be life-threatening.
"The London Ambulance Service and London's Air Ambulance provided first aid; he was pronounced dead at 18:56hrs".
Rhyhiem's mother, Pretana Morgan, paid tribute to her son - an aspiring architect and rapper - and criticised police for "putting us in danger". Enough. My heart goes out to families grieving children and teenagers.
Three people were injured in a "noxious substance" incident after an altercation between two groups in east London at around 5.20am on Sunday.
Police are investigating whether the men, aged 22 and 27, and a 17-year-old boy, suffered life-changing injuries in the incident, at Shacklewell Lane in Hackney.