Because of the effect of longitudinal wall conduction on the thermal performance in a direct transfer-type, single-pass, three-fluid crossflow heat exchanger, the exit mean temperatures and the deterioration factors in each fluid are calculated by the numerical method. Unit construction of the three-fluid crossflow heat exchanger is that fluid 2 is sandwiched between fluid I and fluid 3. The results of deterioration factor versus number of transfer units and the inlet temperature of fluid 3 are explored to demonstrate the influences of different heat capacity rate ratios, heat transfer resistance ratios, and longitudinal wall conduction ratios