Pol Maj Prasert Techama, a case officer at Ban Chang police station, said the attack occured on the second floor of the police station.
About 30 men in naval police uniforms used their batons to assault Charoeanpong Painuen, 30, Sutas Iamchorn, 22, and Terdsak Kruitnoi, 34. The victims were beaten unconscious and later taken to hospital.
Mr Terdsak owns a shop selling mobile phones in a department store in Ban Change district. He had argued with a naval officer, Capt Tinnakorn Kanchanatemeh, on Dec 4, Pol Maj Prasert said.
They were negotiating a settlement at the police station on Thursday.
Capt Tinnakorn had demanded 200,000 baht in compensation from Mr Terdsak, but he offered only 50,000 baht.
Mr Terdsak had brought two friends to the station back him up. The navy officer called in a squad of his men.
Lt Cmdr Pichit Phetcharattanu, 46, was also present in the room. He became annoyed at the lack of progress in the negotiations.
A short time later the navy policemen began beating up Mr Terdsak’s friends, Mr Charoenpong and Mr Sutas, who were waiting outside the room.
Mr Terdsak saw what was happening and walked outside the room, and he was attacked too.
Pol Lt Col Songkran Sanhakorn, deputy superintendent of Ban Chang police station, had tried to intervene and control the situation, but the navy police blocked his efforts.
The three civilians were beaten unconscscious and the navy personnel then quickly left.
Police had tried to stop the navy vehicle, but the driver rammed his way through them and they fled off.
Mr Terdsak has filed a police complaint and investigators will arrest those responsible, Pol Maj Prasert said.