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.
Wednesday night four people surrendered to police and handed over guns. They claimed they shot the men in self-defence.
The suspects are Somsak Songrit of Songkhla, Donwalard Rakrit of Phatthalung and two Krabi men, Surapol Wichaidit and Chatchai Yoydam.
The provincial court issued arrest warrants for the men on murder charges. Police are unconvinced the men acted in self-defence based on the weapons used in the attack.