A microwave three-channel selector, showing twofold symmetry using tri-operational mode synthesized transmission lines, is proposed and investigated in this paper. On one hand, just like a typical triplexer, the three-channel selector provides independent transmission paths at three different frequencies; however, on the other hand, the twofold symmetry makes each of the ports able to serve as the common input. The arrangement of the proposed channel selector is unique and can be applied to synthesize a tri-mode heterogeneous integrated beam-switching/Van Atta/phase-conjugating array. The operational principle is introduced first, followed by the details of the synthesized lines as the core building blocks; design formulas are discussed along with the results. Experimental validation verifies the design concept, and discussions on its potential applications are provided.