The three fatalities were a 40-year-old woman, Wirawan Sibuala, and two 16-year-old youths, Sarawut Utla and Apisit Saengnet, police said.
Permanent Secretary for Public Health Narong Sahametapat was touring health ministry medical services in the northeast, and visited the victims today.
The explosion occurred at 3.40am during a celebration for a new temple, Wat Ban Ta Laew, in Na Yai subdistrict.
After the incident, the Roi Et Provincial Public Health Office dispatched some 10 medical teams to care for the victims at the scene and transferred the injured to hospital, Dr Narong said.
Ten persons -- five men and five women-- were still in hospital, he said, adding that most were superficially wounded by fragments from the grenade, but some suffered broken arms and legs.
Two of the men were seriously injured from the powerful explosion, said Dr Narong.