A new three-phase alloy of 50Cr–30Mo–20Nb (at%) was studied based on the Cr–Mo–Nb ternary phase diagram, and was composed of a Cr-rich BCC₁ phase, a Mo-rich BCC₂ phase and a NbCr₂ Laves phase after heat treatment at around 1523 K or lower. A supersaturated BCC single-phase solid solution alloy obtained by homogenization at 1973 K for 1 h underwent microstructural evolution during heat treatment at 1473 K. Intragranular precipitation of the Cr-rich BCC₁ phase occurred, which led to the formation of an alternating BCC₁/BCC₂ two-phase microstructure through a discontinuous precipitation process, followed by precipitation of the Laves phase at the BCC₁/BCC₂ interphase boundaries. At a higher temperature of 1523 K, a similar microstructure was observed, with increased BCC decomposition and Laves precipitation rates, while the alloy consisted of BCC and Laves phases at 1773 K. The mechanical properties of alloys heat treated at 1473 K for various periods after the solid-solution treatment were also investigated. A maximum fracture strength of 1493 MPa and a minimum hardness of 773 ± 7 HV were obtained for an alloy aged for 24 h, where the BCC₁/BCC₂ two-phase microstructure dominated. The Vickers hardness of an alloy aged for 72 h, which had a fine-grained microstructure that included the Laves phase, was 839 ± 8 HV under a load of 0.5 kgf, and no obvious microcracks were observed.