Kobkul was shot dead in her pickup truck in May while under police protection.

A police officer and Kobkul's driver were injured in the attack.

The suspects were identified as Anont Phanrat, Winyu Rattanawannee and Anantasak Srisawas. In addition to the charge of premeditated murder, they also face charges of illegal possession of guns and ammunition and unauthorised carrying of the weapons in public places.

Anont has pleaded guilty to all charges, while Winyu admitted only to premeditated murder and Anantasak pleaded innocence. All the suspects are in jail pending trial.

The Criminal Court will summon the suspects to give their statements today.

A source said public prosecutors had not yet brought action against caretaker Senator Napinthorn Srisan-pang, alleged to be the mastermind behind Kobkul's murder, because more investigation was needed to prove the allegation.

Napinthorn has maintained his innocence and said he had no business or political conflict with Kobkul.

However, the senator was implicated because one of the gunman admitted to shooting Kobkul to repay Anantasak - a member of Muang Ratchaburi Municipality and the manager of Sri Muang Market, which is owned by Napinthorn.

Public prosecutors said they would announce their decision on caretaker Senator Napinthorn on October 2.