Officers were called to the A13 in Wennington Marshes, shortly after 10am on Monday, 15 July to reports of a collision involving a moving car and a second stationary vehicle.
The stationary vehicle had been involved in a collision a short time earlier.
A fourth person was taken to hospital for treatment and has now been discharged. Their injuries are not life-threatening. No arrests have been made.
DI Julie Trodden, from the Serious Collision Investigation Unit, said: “This is an extremely tragic incident and my thoughts remain with the families of those who lost their loved ones.
“We are keen to speak with anyone who may have been driving along this stretch of road around 10pm on 15 July and who may have dashcam footage of the incident.”
Anyone with information should call the Serious Collision Investigation Unit on 0208 597 4874 or message @MetCC on quoting 8672/15JUL.
The fatal collision occurred after the police had been called to the initial non-fatal collision but before officers arrived at the scene.
The police response to the initial collision is now subject to a mandatory referral to the Independent Office for Police Conduct (IOPC).
While the collision took place within the Metropolitan Police area, it was very close to the border with Essex Police and calls to police following the first collision were answered by Essex’ control room.
As a result, both forces have made referrals to the IOPC.
