A three fluid heat exchanger in which first (fuel) and second (oil) fluids are in a normally flowing heat transfer relation, with flow of a third fluid (compressor bleed air) being dependent on a changing condition of one of the first or second fluids. Tubular heat exchanger cores are comprised in a unitary body with one being perpendicular to the other for space conservation and to simplify fluid flow patterns. The described one core is in the path of flow of one of the first or second fluids and provides a flow path for the third fluid. A valve influenced by a changing pressure condition of one of the first and second fluids controls flow of the third fluid.