A frequency-tripled laser-resonator has three resonator-branches. The branches are optically connected with each other by one or more polarization-selective devices. Unpolarized fundamental radiation is generated by optically pumping a gain-element in one branch of the resonator. The polarization-selective device provides that radiation in the other branches is plane-polarized, with the polarization planes of radiation entering the branches perpendicular to each other. Two optically nonlinear crystals are located in one of the branches of the resonator in which the fundamental radiation is plane-polarized and arranged to generate third-harmonic radiation. Three-branch resonators including two gain-elements having a optical relay therebetween, and a three-branch ring-laser-resonator are also disclosed.