Jenna Barbour, 18, Josh Stewart, 16, and David Armstrong, 15, died when the car they were travelling in crashed into a wall in Tyninghame, East Lothian, in the evening of November 25 last year.
Robbie Gemmell, who was 16 at the time, was the driver of the car and survived the collision.
At Edinburgh Sheriff Court today, Gemmell pleaded guilty to causing death while driving without due care and attention, police said. He will be sentenced on September 19.
The two boys who died were pupils at Dunbar Grammar and Ms Barbour was a former pupil of the school.
Her parents Sandra and Drew and brother Gregor said they were completely devastated by her death.
In a statement released through police shortly after the crash, they said: ''We were all immensely proud of Jenna and everything she achieved in her life. She was always a delight to be around and the family each have very fond memories of our holidays together.''
David's mother Andrea said: ''David was a kind-hearted and good-mannered boy, who was always respectful of adults and his peers and was a loving brother to his sister.''
Josh's parents Susan and Mark said: ''Josh was a much-loved son, nephew, grandson and cousin, and a loyal friend.
''He was a joy to anyone who knew him and the people that met Josh and his smile would never forget him.''
A memorial is being planned for the three teenagers at Dunbar Grammar.
Inspector Richard Latto, the senior investigating officer in the case, said: "This single-vehicle road collision resulted in the deaths of three young people. All were from the Dunbar community and our thoughts are with the families affected by this tragic incident.
"An extensive police inquiry was carried out to determine the cause of this collision, and a detailed police report submitted to the procurator fiscal.
"This tragic incident reinforces the extensive risks of an inexperienced person not legally old enough to possess a driving licence, getting behind the wheel of a motor vehicle."