Two women and a two-year-old child were charred to death after the explosion in the cracker unit caused a fire.
One of four injured persons was stated be serious, Ramachandrapuram DSP S Srinivas said.
A police team led by Srinivas reached the spot and commenced rescue operations. Three fire tenders were pressed into service to control the blaze.
"We received the information (of explosion) at 2pm and rushed to the spot with the police team. We are yet to ascertain the cause of the explosion and details of workers in the unit were not known immediately," the DSP said.
The bodies were yet to be identified, he added.
Meanwhile, the revenue, police and other top officials have reached the spot.