A search of worker shacks and a storage area unearthed bullets.
Police have been stationed in the area pending a decision on what to do with the plantation. Its concession expired on September 18.
The victims are Pol Lieutenant Phayaprasit Suwanmal, who had worked at Thung Song Police Station, and two Phatthalung natives, Weerasak Kuakaew and Preecha Kuadam.
The three were reportedly at the plantation to help the executive of a private company, Chaiyuth Rodkaew, reclaim the 2,000-rai (320 hectare) site from another private company.