The invention relates to a three-cylinder gasoline engine balance shaft mechanism, and belongs to the technical field of vehicle engines. The three-cylinder gasoline engine balance shaft mechanism comprises a balance shaft assembly; the balance shaft assembly and a crankshaft are fixed on an upper cylinder block through fixed brackets; the balance shaft assembly is positioned on the inclined lowerside of one side of the crankshaft; and the balance shaft assembly is engaged with a driving wheel on the crankshaft through a transmission gear. The three-cylinder gasoline engine balance shaft mechanism is compact in structure; through arrangement of the balance shaft on the inclined lower side of the crankshaft, the sizes of engines in both width and height directions are both considered; as aback end gear of the balance shaft drives a machine oil pump to operate, the difficulties of complicated front end driving wheel system and complicated space arrangement are solved; through direct equal-diameter engagement between a front end transmission gear and a crankshaft driving wheel, the front end space arrangement is optimized; the friction power increment caused by the balance shaft isreduced; through the design of a rolling needle bearing and the design of full enveloping of the balance shaft by a lower cylinder block and an oil stop plate, the friction power increment caused by the balance shaft is reduced; and a thrust structure is simple and reliable to prevent the blockage risk.