The youth, now 15, was driving a stolen Range Rover when it was in collision with a minibus. He admitted three charges of causing death by dangerous driving and one of taking the vehicle without consent.
He appeared for sentencing at Winchester Crown Court alongside his passengers, both aged 16. They were given one year's detention after admitting allowing themselves to be carried in the stolen vehicle.
The driver had only ever driven for 20 minutes before taking the V8-engine Range Rover, the court heard.
Mrs Justice Janet Smith said he had driven on to the A3(M) in Hampshire at about 70mph and lost control of the vehicle. It swerved across the central reservation and landed on its roof. Moments later it was hit by a minibus driven by Peter Mitchell. The 41-year-old had been unable to avoid the collision, but had probably saved more lives by trying to swerve, Mrs Justice Smith said.
Mr Mitchell, of Portsmouth, and passengers Ann Williamson, 30, and Doris Hale, 41, died after the crash on 4 January this year. Six other factory workers in the minibus survived.
The teenage driver was banned from driving for five years; his passengers were disqualified for three years.
Iain Lawrie, defending the driver, who is from the Portsmouth area, said his client's guilt would be his "constant and daily companion for the rest of his life".
Outside the court Kevin Vicars, the 24-year-old fiance of Miss Williamson, said he had hoped the driver would have faced a longer punishment.