Fire officials said the workers were painting the interior of the tanker at an STX Offshore & Shipbuilding plant in the southeastern city of Changwon and died almost instantly.
The workers in their 30s to 50s were painting inside a 12-meter-deep oil tank at the time of the accident. The fire agency recovered the bodies of the three victims.
The oil carrier with the capacity of 74,000 tons is reportedly scheduled to be delivered to a Greek shipping company in October.
According to Yonhap, the authorities are looking into the cause of the explosion.
The 74,000-tonne oil tanker was scheduled to be delivered to a Greek shipping company in October, according to Yonhap. South Korea is one of the world’s largest shipbuilders.