In the framework of the constant intensity approximation, we analyze a nonlinear quasi-phase-matched process of frequency tripling, where the second harmonic wave propa-gates in the backward direction. The efficiency of third harmonic generation and second harmonic backward wave are calculated analytically for various orders of quasi-phase matching. Studies of quasi-phase-matched harmonic generation in PPLN crystals, con-ducted under the constant intensity approximation, have shown that increasing the order of quasi-phase matching leads to an increase in the optimal domain thickness. According to the analysis based on the constant intensity approximation, it has been found that the accuracy in determining the quasi-phase-matching periods of an optical superlattice depends on several factors: the precision of the refractive indices at the frequencies of the interacting waves for the specific sample, the second-order nonlinear susceptibility of the domains, and the intensities of the interacting waves. It is shown that the initial phases of the interacting waves have a significant impact on the efficien-cy of frequency conversion. Optimal phase values are identified, demonstrating that proper phase selection can substantially enhance the conversion efficiency. Based on this analysis, design recommendations are provided for optimizing the structure of the optical superlattice to achieve maximum efficiency. These results are of practical inter-est for the development of devices based on third harmonic generation when backward second harmonic is simultaneously generated.