A novel concept and approach for integration of a three-state diplexer (TSD) by utilizing the triple-mode cavity resonators is proposed in this paper. The proposed TSD has the inherent nature of three frequency channels, two of which can be utilized for duplexing channels by exciting one port as the input in each state. Meanwhile, three fundamental modes, namely, TE011, TE101, and TM110, are excited in the triple-mode resonators (TMRs) to control these three frequency channels. Without utilizing any junction networks for impedance matching, the required values of the external quality factors and coupling coeff cients can be extracted to meet the relevant Chebyshev responses in the three bands of the TSD, simultaneously. Owing to the modal orthogonality among the three fundamental modes, high isolation among these three frequency channels can be effectively achieved. For proof of concept, two design examples of the TSD structures are presented with different topologies. For both examples, four and six TMRs are adopted, respectively, where the predicted three frequency channels of the TSD can be successfully excited with excellent channel isolations. The fabricated prototype shows good agreement between the simulated and measured results verifying the feasibility of the proposed design methodology.