The development of test mass with ultralow magnetic susceptibility (χ < ${10}^{-6}{\mathrm{cm}}^3/\mathrm{mol}$) is critical for gravitational-wave detection. Binary $\mathrm{Au}\mathrm{Pt}$ alloys have been identified as promising candidates, though their mechanical, thermal, and electrical properties require further improvement. Introducing a third metal element represents a promising strategy for enhancing their overall performance. In this work, we employ the virtual crystal approximation (VCA) to model ternary $\mathrm{Au}\text{−}\mathrm{Pt}\text{−}\mathrm{Pd}$ alloys and optimize their structures. Using first-principles calculations within the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) framework, we systematically investigate the magnetic susceptibility and its composition dependence in $\mathrm{Au}\text{−}\mathrm{Pt}\text{−}\mathrm{Pd}$ alloys. Our results show that (i) the alloy retains ultralow magnetic susceptibility (χ < ${10}^{-6}{\mathrm{cm}}^3/\mathrm{mol}$) across a wide composition range, notably around 82.0–85.0 at. % $\mathrm{Au}$ and 3.0–5.0 at. % $\mathrm{Pd}$; (ii) the three-dimensional magnetic phase diagram reveals nonlinear hybridization behavior of magnetic susceptibility, with a stable contour of nearly zero susceptibility (χ ≈ 0) near ${\mathrm{Au}}_{82.8}{\mathrm{Pt}}_{14.2}{\mathrm{Pd}}_3$, indicating that $\mathrm{Pd}$ doping can synergistically improve magnetic properties; (iii) this theoretical study elucidates the microscopic mechanisms, electron transfer, and orbital decoupling, underlying the ultralow magnetic susceptibility in multicomponent alloys, and provides quantitative guidance for the design of test masses in next-generation gravitational-wave detectors and related applications, such as medical implant materials.

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• Diamagnetism
• Gravitational wave detection
• Magnetic phase transitions
• Magnetic susceptibility
• Paramagnetism
• Transition metal alloys
• Gravitational wave detectors
• Korringa-Kohn-Rostoker method