In this work we present a model to elucidate the unidirectionality of the primary charge separation process in the bacterial reaction centers (RCs) where two symmetric ways of electron transfer (ET), starting from the common electron donor, are possible. We have used a model of three sites/molecules with ET beginning at site 1 with an option to proceed to site 2 or site 3. The dependence of the asymmetry in ET on the value of the sink parameter, introduced through an additional imaginary diagonal matrix element of the Hamiltonian, was investigated. Results show indeed that the unidirectionality of the ET generated in the system of three molecules depends strongly on the value of the energy levels of the accessory bacteriochlorophyll molecules.