In contrast with two stream heat exchangers, the three stream ones owns much more complexity in design and rating and a comprehensive rating method has not been proposed, yet. In this paper, a new rating method is presented in order to be used in three stream plate – fin heat exchangers. Rating, in heat exchangers, means finding out the heat load of the heat exchanger or in the other words calculating the outlet temperature of each stream and furthermore, finding out the pressure drops of each stream. In this paper, the differential equations of energy conservation and heat balance of each steam is extracted and a set of differential equations in terms of outlet temperatures in constructed. The general solution of this set of equations is then extracted. In order to expand the general solution to plate – fin heat exchangers, the geometric, thermal and hydraulic modeling are applied based on fin geometric specification which used rectangular type in this paper without losing generality. Based on all of the extracted equation, a comprehensive algorithm of rating of the three stream plate – fin heat exchangers is presented. In order to evaluate the presented algorithm, a case study is used and the results will be shown. Furthermore. A parametric study is applied to the case study to evaluate the effects of the geometric parameters of the fins such as height and frequency on heat load and pressure drop of the heat exchanger.