Ten fire engines and around 70 firefighters were called to a house fire on Halesowen Road in Morden.
The ground floor, first floor and roof of a two-storey end-of-terrace house were destroyed by fire. In a neighbouring property, the whole of a ground floor lean-to and roof were damaged in the blaze. And the roof of a third house was also completely destroyed.
One woman, one man and a boy suffered from smoke inhalation and were treated on scene by London Ambulance Service and taken to hospital.
Firefighters used a 32-metre turntable ladder as a water tower to extinguish the fire from above. Crews also cooled one gas cylinder to an ambient temperature, as some cylinders can explode when exposed to heat. It was removed and placed in a safe location.
The Brigade's 999 Control officers received the first of 18 calls alerting them to the fire at 0404. They deployed crews from Sutton, Mitcham, Wimbledon, New Malden, Wallington, Tooting, Norbury, Wimbledon and Clapham fire stations to the scene. The fire was under control by 0606.
The cause of the fire is under investigation.