Open loop ring resonators are used as the basic block of a compact planar triplexer, they are used in cascaded trisection (CT) filters, in order to reduce the circuit size. The CT filter has a cross coupling that produces a single attenuation pole at finite frequency used to shape the bandpass response. Ring resonators are properly located with respect to the input and output feed lines, in order to get good isolation response, and also to suppress channel interfering and to meet the design specifications for applications in mobile communication. The triplexer was fabricated and measured to verify the viability of the design with good agreements between the measurements and the simulations. © 2013 IEEE.