Shortly before 10pm North Wales Police received a report of a road traffic collision on the A499 in Llanbedrog which involved a blue coloured Ford Fiesta ST which had collided with a wall.
Emergency services attended and the three teenagers, who were travelling in the car, were taken to hospital with serious injuries.
One was airlifted from the scene to hospital in Stoke and two were taken to Ysbyty Gwynedd in Bangor by ambulance.
One of the teens has has since been transferred from Bangor to Stoke.
Sergeant Jason Diamond of the Roads Policing Unit is appealing for witnesses and information.
He said: “I would urge anybody who may have witnessed the collision, or who may have seen the blue Ford Fiesta ST being driven prior to the collision to come forward.
“We are also keen to speak to anybody who may have been in the area and who may have dash cam footage to contact us.”
Anybody who witnessed the collision, has relevant dashcam footage or has any information which could help the investigation is asked to contact us via the website or by calling 101, quoting incident number 22000262712.