Police found the body of driver Wannapot Jindarat, 74, in the burning cabin of the truck, following the attack at about 10pm.
Two injured passengers who fled the truck were rushed to a hospital.
The three victims had bought fish from Muang district and were going home to Sai Buri when they reached the U-turn point in Tambon Manang Dalam.
Gunmen on motorcycles suddenly shot at the truck with an AK assault rifle and poured fuel on the truck to burn them.
Wannapot was killed while passengers Noppasin Foythong, 20, and retired policeman Jehloh Yako, 64, managed to flee the burning vehicle.
Police blamed the attack on the southern separatists.
-The Nation/Asia News Network