A 28-year-old man was arrested on suspicion of dangerous driving after a black Audi, believed to be travelling at high speed, mounted the pavement near a bus stop on Shepiston Lane, Hayes.
The three victims have been named locally as Harry Rice, George Wilkinson and Josh Kennedy, who are understood to have been on their way to a birthday party at a nearby football club.
The suspect was questioned at a north London police station on Saturday night, the Metropolitan Police said.
The Met have now said they believe at least one other male travelling the Audi fled the scene of the crash.
The second male has yet to be apprehended and investigators have not ruled out more passengers being in the car.
"Enquiries are ongoing to establish the full circumstances of the collision and the number of occupants in the vehicle involved," the Met said in a statement.
Emergency services were called at 8.41pm on Friday, but despite the efforts of paramedics, all three victims were pronounced dead at the scene.
Two were aged 16 and one was 17.
Staff at the nearby Esso garage said a group of teenagers had been fighting near the entrance to the forecourt at about 8.40pm.
One cashier said the police had come to inspect CCTV footage of the brawl, but said he did not know if it was related to the crash.
Jake Webb, 18, saw two white youths attacking an Asian man on the side of the road near the scene as he was driving towards Hayes and pulled over his car.
“I said, ‘Can you stop that, we’re going to call 999’,” he said. “They said, ‘Please call the police. He’s hit our friend’.”
Mr Webb’s friend, Amamdeep Matharoo, 27, said: “The lads were shouting on the phone. They were saying, ‘three of our friends are dead’. They were hysterical and in pieces.”
One tribute left at the scene read: "George Boy, I love you so much, love Kev, Vinnie."
Another said: "I thought I would never have to write this and I don't know what to write but I will always love and miss you millions."
Elsewhere, a card read "To my bro, always together forever," while a fourth said: "To my darling baby boi my heart is broken, love mum, Vinnie."
Detective Superintendent Jane Shaw, of the Met's Serious Collision Investigation Unit, said: "These are horrific circumstances that have resulted in three young people on a night out losing their lives."
She added: "We know that there was a large party nearby and many people would have been travelling through the area and saw what happened or witnessed the aftermath.
"I need these witnesses to contact police immediately.
"We are also particularly keen to trace other motorists travelling through the area at the time."
Additional reporting by Press Association
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies