Four people were injured when up to five males “indiscriminately” fired a weapon into a large crowd during the event in Clarence Gardens on Saturday.
Officers arrested a 22-year-old man on suspicion of possession of a firearm with intent on Tuesday, while two boys, aged 16 and 17, were arrested on Wednesday night.
The 16-year-old was arrested on suspicion of possession of an offensive weapon and robbery, as well as possession of a firearm with intent. The 17-year-old was held on suspicion of possession with intent to supply and possession of a firearm with intent.
Shots were fired into a group of people attending the communal barbecue near Regent’s Park just before 10.10pm on Saturday, the Metropolitan Police said.
Firearms officers were sent to the scene, along with paramedics.
The injured – a 19-year-old woman, a 17-year-old girl, a 28-year-old man and a 73-year-old woman – were taken to hospital.
One was later discharged and the others are not thought to be in a life-threatening condition.
The Specialist Crime North Reactive team is investigating the incident.
Detective Sergeant Val John-Baptiste said: “I need to hear from anyone who witnessed the incident or saw anything suspicious in and around Clarence Gardens, NW1.
“We thoroughly investigate every shooting in London – every incident and every weapon on our streets is of great concern to us.
“We know that Londoners feel exactly the same, and that’s why it is vital that anyone with information shares with us what they know, either directly with police or anonymously via Crimestoppers.”