The engine cooling radiator and the evaporator of the air conditioning system for a vehicle are combined in a single triple heat exchanger, in which the refrigerant fluid and the engine coolant fluid flow respectively in two sets of flat pockets which are stacked alternately with gaps in which the air to be treated flows. A pocket of one set is directly juxtaposed to a pocket of the other set, to form a pair of pockets, between each gap and the next, to give direct heat transfer between the refrigerant and engine coolant fluids.