An air heating, air conditioning and water heating system includes a multi-communicative valve unit, a compressor arranged for compressing the refrigerant in a state of superheated vapor, a condenser communicated with the compressor through the multi-communicative valve unit, a heat exchanger communicated with the condenser through the multi-communicative valve unit, an expansion valve, and a water heater communicated with the heat exchanger and the compressor through the multi-communicative valve unit, wherein the multi-communicative valve unit is arranged to be operated to selectively establish at least an air conditioning route, an air heating route, and a water heating route for the refrigerant so that the air heating, air conditioning and water heating system is capable of selectively providing air conditioning, heating and delivering hot water for a predetermined premises.