We present a bottom-up fabrication route to fabricate solid-solution Rh–Pd–Pt ternary alloy nanoparticles (NPs), with well-controlled compositions through femtosecond laser irradiation of a mixed solution of metallic ions, without any reducing agents and complicated processes. The structure of fabricated NPs was crystalline, and formation of solid-solution alloy formation was confirmed by electron and x-ray diffraction measurements. The crystalline nature of alloy NPs was also confirmed through high-resolution transmission electron microscope measurement. According to energy dispersive spectroscopy analysis, elemental composition of an individual NP was almost the same as the initial feeding ratio of ions in the mixed solutions. The electronic state of the element in alloy NPs was confirmed to be pure metal by XPS measurement. The structural studies of Rh–Pd–Pt NPs suggested that the demonstrated technique opens up a new dimension for the fabrication of NPs, which has well-controlled properties for practical use in various fields.