Formation of Au-Ag-Cu ternary alloy nanoparticles (NPs) is difficult mainly because the system Cu/Ag is immiscible. We present a new synthetic technique to generate such ternary alloy NPs in silica film matrix employing a three-layer (3L) coating design. In this methodology three successive coating layers were deposited on silica glass substrates from separately prepared Cu-, Au- and Ag-ion incorporated inorganic-organic hybrid silica sols by dipping method. The Au layer is kept in the middle because it is miscible with both the Ag and Cu. The 3L film assembly was subjected to UV- and heat-treatment at 450-750 degrees C in H2-N2 atmosphere. UV-treatment generates small Au and Ag NPs in the respective layers and Cu remains as Cu2+; subsequent heat-treatment in H2-N2 induces the formation of ternary alloy NPs by the interlayer diffusion of nanometals. The final heat-treated film (750 degrees C/H2-N2) shows single and sharp plasmon band centered at 480 nm in the UV-visible spectrum indicating the formation of alloy (solid solution) NPs. GIXRD study shows one set of diffraction peaks which are shifted towards higher angle with respect to the Au or Ag diffraction peaks. FESEM, GIXRD, HRTEM, and SAED analyses reveal that the alloy has a composition close to (Au + Ag)0.88Cu0.12. The EDS analyses using the nano probe attached with TEM confirm the presence of Au, Ag and Cu in all the alloy NPs.