Four fire engines and around 20 firefighters and officers were called to a fire at a house in Parkway in Southgate.
Part of the ground floor of the detached house was damaged by fire.
Two teenagers left the property before the arrival of the Brigade and crews wearing breathing apparatus rescued a third teenager by ladder from first floor level.
All three teenagers were both treated at the scene for smoke inhalation by the Brigade and London Ambulance Service crews before being taken to hospital.
Watch manager Wayne Healey, who was at the scene, said: “When crews arrived, the whole of the garage was alight and firefighters worked hard to stop the fire from spreading from the garage to the rest of the property. We had to rescue one teenager from the first floor as he became trapped due to smoke."
The Brigade was called at 2048 and the fire was under control at 2159. Fire crews from Southgate, Edmonton and Barnet fire stations attended the scene.
The cause of the fire is under investigation.