Solid-phase equilibria in the GeSb2Te4-SnSb2Te4-Sb2Te3 subsystem of the GeTe-SnTe-Sb2Te3 system were investigated by powder X-ray diffraction method. Alloys were synthesized from previously prepared binary compounds by melting, quenching from the liquid state, and subsequent annealing at 800 K for 800 h. It was established that the GeSb2Te4-SnSb2Te4 and GeSb4Te7-SnSb4Te7 sections form continuous series of substitutional solid solutions with tetradymite-like layered structures. In both systems, the lattice parameters increase systematically with increasing Sn content, consistent with Vegard’s law. In contrast, the GeSb6Te10– “SnSb6Te10” section is characterized by limited solid solubility. Based on the experimental results and literature data, the solid-phase equilibrium diagram of the GeSb2Te4-SnSb2Te4-Sb2Te3 system at 300 K was constructed. The diagram contains four single-phase regions, four two-phase regions, and one three-phase region. The obtained results provide new information on phase relations and compositional stability in the Ge-Sn-Sb-Te system and are useful for the development of functional layered chalcogenide materials.