West Yorkshire Fire and Rescue Service said it is working alongside police, ambulance, the local council and utility companies following an explosion at a home in Green Lane, Illingworth, near Halifax.
The three people have been taken to hospital, and the fire service said all other persons have been accounted for.
Six nearby houses have been evacuated as a precaution.
A statement from the fire service said: "The incident was initially attended by fire engines from Fairweather Green, Halifax and Odsal with specialist units from Cleckheaton and Dewsbury.
"Crews are dealing with any remaining pockets of fire and they are being extinguished should they appear. Firefighters are likely to be on the scene for some time.
"Investigations are continuing into the cause of the explosion."
A tweet from Northern Gas said: "Our engineers are currently attending an incident in Green Lane, Illingworth Halifax. They're working with West Yorkshire Fire and Rescue Service, West Yorkshire Police and Calderdale Council to ensure that the area is safe and to determine the cause of the incident. Anyone that smells gas should call 0800 111 999."