Recently, alloys have been attracting attention as plasmon resonance materials, e.g., Au, Ag, and Cu alloys were made . We obtained, for the first time, the complex permittivity ε ≡ (n + iκ) 2 of a metallic tri-metal alloy made of Au, Ag, and Cu at different mixing ratios and used those alloys for their plasmonic function (n is the refractive index, and κ is extinction). We also found that forming alloys with Au and Pd improves the hydrogen response and enables the construction of optical hydrogen sensors. In optical applications based on plasmon resonance, it is crucial to determine the complex refractive index (n + iκ) of metallic materials. This is essential for electromagnetic field calculations, including numerical methods, e.g., finite differences time domain (FDTD) calculations which provide exact numerical solutions of Maxwell's equations. However, it is known that when an alloy is formed, its complex permittivity becomes different. It is not accurate to use the arithmetic mean of the constituent part to describe the alloy . In addition, the permittivity changes because of the size and defects (grain boundaries, stacking faults, dislocations, porosity) depending on the film's deposition conditions. Therefore, evaluating the permittivity of a film deposited under the exact same conditions used for nano-particles top layer of MIM is always required, and ε has to be determined experimentally.