The three major gas stations refer to

China Petroleum, China Petrochemical and China Offshore Oil. China Petroleum is one of the largest oil and gas producers and "suppliers" in China and one of the largest oil companies in the world. China Petrochemical is one of the largest oil refining enterprises and petrochemical producers in China, and also one of the largest sellers of refined oil products in China. China Offshore Oil Corporation is the largest offshore oil and gas producer in China and the third largest oil company in China. So it's China Petroleum, China Petrochemical and CNOOC China.