The 21-year-old man suffered a punctured lung and serious damage to his windpipe.
He was taken to a south London hospital, where he received immediate surgery but subsequently made a full recovery.
The 16-year-old suffered a minor wound to his arm.
The Met's gang unit Trident has opened an attempted murder investigation into the attack which happened at around 7pm on May 12 on Marcella Road in Brixton.
One of the suspects grabbed a bike and cycled off, with his two accomplices running behind, trying to mask their faces with bandanas.
The suspect on the bike can be seen in the footage waving the gun in the direction of two passers-by just yards away as he cycles past making his escape.
Detective Constable Jo Ross said: "The 21-year-old victim was fortunate to recover from what were potentially life-threatening injuries at the time of the incident.
"Other passers-by were threatened by having a shotgun pointed at them - these are dangerous people.
"We are keen for the public to give us any information in relation to this appeal, who witnessed the incident or who recognises the suspects. It happened in broad daylight and in an area full of schoolchildren."
Police also offered the following description of the three attackers:
Anyone with any information is asked to contact police on 101 or Crimestoppers anonymously on 0800 555 111.
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