The bomb, believed to have been triggered by a remote control, had been hidden under a bench and exploded at the event held on Wasukree beach in the southern town. The three officers in the area were killed.
They were identified as Pol Lt Col Charnyuth Kaenpetch, Pol Lt Col Surasith Charoenphon and Pol Lt Col Apirak Saefu.
About 15 others nearby were injuried, including five police officers, Sai Buri police said.
Authorities have been ordered to stay alert and are taking strict security measures amid concern by the army and the National Security Council about possible major attacks by militants this month.
This coming Monday is the 10th anniversary of the Kru Se massacre in Pattani's Muang district.
The incident began when 32 suspected guerrillas took shelter in the Krue Se mosque. After a seven-hour standoff with Thai military personnel, soldiers attacked and killed all 32.
The attack was carried out in contradiction of orders from the defence minister to end the confrontation peacefully.
The Krue Se incident sparked the revival of a long-dormant insurgency movement and a decade of continuing violence that has taken nearly 6,000 lives.