Eight fire engines and around 60 firefighters were called to a fire on Hoe Street in Walthamstow.
A single-storey building at the rear of a shop was completely destroyed by the fire.
Three men were rescued from the building by fire crews wearing breathing apparatus. They were treated at the scene by London Ambulance Service crews and the Helicopter Emergency Medical Service and taken to hospital.
The Brigade was called at 0558 and the fire was under control by 0820. Fire crews from Walthamstow, Leyton, Tottenham, Woodford, Homerton and Bethnal Green fire stations attended.
The cause of the fire will be investigated by the Brigade and the Metropolitan Police Service.