Six different effectiveness parameters or figures of merit are defined based on the five identified engineering goals of three-fluid heat exchangers having three thermal communications. The effect of six non-dimensional design parameters on the defined figures of merit is discussed. It is shown that various effectivenesses of this class of heat exchangers strongly depend on the relative thermal capacities of three fluid streams. Any single definition is shown incapable of incorporating all of the five different objectives. A distinction is made between the thermal and temperature effectivenesses of heat exchangers. (C) 2004 Elsevier Ltd. All rights reserved.