Police said the fire started at about 7.30am in a three-storey building in Songkhla's Sadao district. Firefighters took about two hours to put out the blaze.
The fire killed Rangsima Jaima, her one-year-old daughter Athicha Premsilpa and karaoke bar owner Akaramanee Jaima.
Six people were injured in the blaze. One of them suffered a broken leg after jumping off the third floor of the building.