Those killed included two women, while four other workers were wounded and taken to hospital.
The incident occurred on the outskirts of Naivasha town during which an unknown amount of money was stolen, according to police.
Naivasha Police Chief Charles Kortok said the suspects were heavily armed when they raided the fuel station.
Two similar fuel station raids occurred in Nairobi at the end of last year.
In one of the incidents on Outering Road, four fuel station workers were bludgeoned to death on December 29, in what is believed to have been orchestrated by the same gang or their accomplices.