A heat exchanger apparatus is provided wherein a multifluid or at least three-fluid heat exchanger is mounted externally to but in combination with a two-fluid heat exchanger. The multifluid heat exchanger includes three fluid passages or conduits wherein heat energy can be transferred efficiently between at least one of the fluid conduits and each of the other fluid conduits. The multifluid heat exchanger and two fluid heat exchanger are arranged so that the two heat exchangers share a common fluid, the multifluid heat exchanger, therefore, allowing heat transfer to or from the common fluid to the two other fluids in the multifluid heat exchanger thereby improving the overall heat transfer amongst the fluids.