The utility model discloses a three -way pipe, heat exchanger and air conditioner, wherein the heat exchanger includes three -way pipe and a plurality of heat exchange tube, the three -way pipe include first pipeline and simultaneously with the second pipeline and the third pipeline of first pipeline intercommunication, second pipeline and third pipeline are used for the influent stream simultaneously or effluent, and the internal diameter of third pipeline is less than the internal diameter of second pipeline, second pipeline and third pipeline all are connected with the heat exchange tube. The utility model discloses technical scheme can improve the heat exchange efficiency of heat exchanger.