They were killed while they were praying at a mosque in the Saba Yoi district of Songkhla province, which is near Yala.
"The victims were all men in their 20s. They were killed inside the mosque," Acra said, but did not give further details.
The deadly attacks capped a day of violence in Thailand's strife-torn south, where two other people were shot dead and nine soldiers injured in bomb blasts earlier on Thursday.
Thursday's violence came as thousands of protesters in the capital, Bangkok, demonstrated against Thailand's junta after a court barred Thaksin Shinawatra, the former prime minister, and his Thai Rak Thai party from contesting elections.