The accident happened at 2.30am on the Sattahip-bound Sukhumvit road in Tambon Bang Lamung of Bang Lamung district.
Rescuers spent more than half an hour using iron cutters to remove the victims from the wreckage of the car.
The three passengers were identified as Thongphet Pekon, 53, Chamras Phothibat, 51 and Charassaeng Wichachai, 45. The driver did not carry any ID card and has yet to be identified.
The truck driver, Prayad Khurasi, 60, said he was changing from the left to the right lanes to make a U-turn when his truck's rear was hit.