Fire service officials said that the gas originating from the septic tank caused the fire.
The three victims — Monowara Begum, 45, daughter Zahida Akter, 20 and son Zakir Hossain, 14 — have been admitted to the MAG Osmani Medical College and Hospitals.
Doctors say the mother and daughter are in critical condition with almost 60 percent burns.
Sylhet Fire Service’s Station Officer Jabed Hossain Tarek said that the incident occurred around 11:30pm on Friday.
Locals had put out the flames before the fire service reached the spot, he said.
Gas from the septic tank, which had leaked , caused the fire, according to the fire officer.
“The daughter went to the toilet with a candle as there was a power cut. The fire broke out as soon as the commode’s lid was lifted.”