As a component for microwave-circuits in tri-plate-line-technique, especially circulators, equilateral triangles are studied. For the lowest modes resonance frequencies and characteristic impedances are given. For both of them the influence of the fringing field is estimated. In comparison with the circular- and square-shaped resonator the triangular resonator has the highest characteristic impedance at the fundamental resonance. For instance at circulators this means more bandwidth than using circular-shaped resonators.