A 23-year-man was arrested at the scene of the incident on Saturday (3 December) and police say that the motive for the killings is unclear. The local politician shot dead was named as Tiina Wilen-Jappinen, the chairwoman of Imatra's town council. She had represented the Social Democratic Party on the city council since 2001, according to local media.
Saku Tielinen, from the Southeastern Finland Police Department, who is leading the investigation, said the suspect had a criminal record, including for violent incidents and theft but did not identify him or give further details.
"There is no indication of a political motive. It seems the gunman and victims didn't know each other," Tielinen said. "All indications are that he chose his victims randomly."
"At the moment, it looks like the victims were chosen at random," Detective Inspector Saku Tielinen said.
Two of the victims were in their 50s while the other was in her 30s, police said in an AP report.
The suspect will be questioned on Sunday (4 December). According to police, the alleged killer waited in his car outside the restaurant and shot the three women when they left the building.
The victims died instantly. The 23-year-old man did not resist arrest but accompanied officers peacefully and without speaking to the police station. He had no alcohol in his system and police are carrying out a blood test and will release the results soon, said South Lake Saimaaa journal.
Saara Raudasoja, a spokeswoman for the South Karelia Social and Health Care District, said: "We decided to open a crisis centre after the shooting occurred right in the middle of the town near restaurants and nightclubs.
"There were people there when it happened. In such a small place, it's quite a huge incident and many people were shocked."
Just 28,000 people live in Imatra, which is about 140 miles (230km) from Finland's capital Helsinki.