A fourth person was admitted to a hospital. The tanker truck's driver and two onlookers rushed to rescue a sanitation worker who had fainted while cleaning the tank.
All four were poisoned by the tank's toxic fumes, a spokesman of the local government said.
Firefighters took the four to a local hospital for treatment following the incident.
The local fire department said proper equipment and protective outfits are required for cleaning chemical tanks, adding that car wash stations do not typically possess this equipment.
