Songkhla governor Kritsada Boonraj said the first two bombs, hidden in motorcycles, were triggered at Padangbesar checkpoint and within the compound of Sadao Police Station. No one were wounded in these incidents.
Casualties were however reported at the third crime scene near Oliver Hotel, he said, adding that 26 people were wounded by the blast. They were rushed to nearby hospitals.
The bomb was planted in a car left parked near the hotel. The explosion also damaged commercial buildings in the area.
The governor said related officials are investigating the motivation behind the attacks, whether it was insurgency or conflicts related to illegal goods smuggling.
Mr Kritsada said he has instructed security officials to step up security measures as the New Year celebration period approaches and there will be a number of both Thai and foreign tourists visiting Sadao and Hat Yai.