Two of them, who sustained burn injures, were taken to a hospital while one was injured less severely, sources at the company said.
Operations at the bio-oil plant were halted and it was likely to remain closed for several days, according to plant manager Timo Partanen.
The rescue department received alarm at around 2pm. The gas explosion happened after the workers had gone to inspect a tank.
The injured workers managed to flee the scene of explosion by themselves.
The plant was commissioned last year to produce bio-oil from wood-based raw materials.
