In this paper, a three ringresonator serially coupled is considered as an optical filter. We are going to improve the performance of the designed optical filter by increasing the quality factor and finesse of filtered wavelengths. The first and last rings are coupled to the bus waveguides that carry the input and output fields. The effect of coupling parameters and ring radii on the filtering of operating wavelengths which are between 1545-1550 nm with narrow Free Spectral Range (FSR) less than 0.5 nm is investigated. Using the transfer matrix method, all the rotating and output fields are obtained. FSR, Full Width at Half Maximum (FWHM) and Finesse (F) are evaluated by the wavelength response plots of the output ports obtained in Wolfram Mathematica. The behavior of structure is analyzed by a new approach in order to filter the resonant wavelengths of the transmission channel with higher finesse.