Four fire engines and around 25 firefighters were called to a fire on Oldfield Lane South in Greenford.
Part of a flat on the first and second floor, above a shop, was damaged by the fire. One man was taken to hospital by London Ambulance Service crews suffering smoke inhalation and another two men were treated on scene.
The fire was discovered when a neighbouring shop worker smelled burning and saw smoke.
The Brigade was called at 0950 and the fire was under control by 1124. Fire crews from Southall, Ealing, Northolt and Wembley fire stations were at the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.