Hcp–Ir0.24Ru0.36Os0.40 and fcc–Ir0.84Ru0.06Os0.10 ternary alloys as well as binary hcp–Ir0.33Ru0.67 and fcc–Ir0.75Ru0.25 ones were prepared using thermal decomposition of [IrxRu1-x(NH3)5Cl][OsyIr(1-y)Сl6] single-source precursors in hydrogen flow below 1070 K. These single-phase alloys correspond to ternary and binary peritectic phase diagrams and can be used as synthetic models for rare iridosmine minerals. Thermal decomposition of parent bimetallic precursor [Ir(NH3)5Cl][OsСl6] has been investigated using in situ powder X-ray diffraction in inert and reductive atmospheres. In reductive atmosphere, [Ir(NH3)5Cl][OsСl6] forms (NH4)2[OsСl6] as crystalline intermediate; Ir from its cationic part is reduced by hydrogen with a formation of defect fcc-structured metallic particles; the final product is a metastable hcp–Ir0.5Os0.5 alloy. In inert atmosphere, the salt decomposes at higher temperature without a formation of any detectable crystalline intermediates; two-phase fcc+hcp mixture forms directly above 800 K. Room temperature compressibility up to 50 GPa has been studied for all prepared alloys in diamond anvil cells. Investigated ternary and binary alloys do not show any phase transitions upon compression at room temperature. In contrast with other investigated ultra-incompressible refractory alloys with osmium and iridium, hcp–Ir0.33Ru0.67, fcc–Ir0.75Ru0.25 binary and fcc–Ir0.84Ru0.06Os0.10 ternary alloys show higher compressibility in comparison with pure metals. Fcc–Ir0.75Ru0.25 alloy shows several magnetic phase transitions (at approx. 3.4 K, 135 K and 233 K) that could be related to different magnetic phases.