<p>Traditional metal alloys require energy-intensive processes to manage atomic complexity for improved strength and toughness, with stringent control over impurities and processing conditions. High-/medium-entropy alloys (HEAs/MEAs) offer a sustainable alternative by introducing atomic complexity with fewer constraints, enabling unique phase transitions. Here, a Cu–Au–Ag MEA is fabricated using vacuum suspension melting followed by heat treatment. The alloy exhibits three distinct levels of heterogeneous structures: micron-scale phase separation, nano-ordered L1₂ phase, and chemical short-range order (CSRO). It demonstrates tensile and compressive strengths of 550 MPa and 1700 MPa for the micropillar sample, respectively, with 25% tensile elongation and over 50% compressive strain. The large-scale sample achieves a tensile strength and elongation of about 500 MPa and 40%, respectively. Compared to traditional gold, silver, and copper alloys, both the strength and plasticity are enhanced. The microstructural characteristics and corresponding mechanical properties are found to match the experimental structure through high-temperature ternary alloy phase diagrams and simulations. This work introduces a alloy design approach that leverages atomic affinity manipulation to regulate multilayer heterogeneous structures, which offers an efficient pathway for designing MEAs suitable for demanding high-performance electronic packaging applications.</p>