The shooting occurred in the shipping company office in Glyfada, a seaside suburb of the capital, Irish News reports.
The alleged gunman, an unidentified company employee, entered the building armed with a shotgun and handgun, according to several witnesses.
Police spokesperson Constantia Dimoglidou said the owner of the company was among the victims.
“The suspected perpetrator, who worked for the family, was found dead in a different area from the three deceased people at the company. He had his assault weapon next to him and appears to have shot himself in the head,” Ms Dimoglidou said.
She said police and special forces officers had entered the building before the suspected gunman was found and had been making sure other employees were moved safely out of the building.
MNA/PR