Trimetallic alloy nanoparticles (NPs) hybridized with graphitic carbon nitride (g-C3N4) nanosheets show potential as photocatalysts owing to their multicomponent interactions, increased catalytic efficiency, and enhanced stability. Herein, a NiCoAg-CN nanocomposite comprising NiCoAg alloy NPs deposited on g-C3N4 nanosheets was synthesized via photoreduction using simulated solar light. X-ray photoelectron spectroscopy revealed the metallic properties of Ni, Co, and Ag within the NiCoAg-CN nanocomposite, and the high-resolution transmission electron microscopy image showed alloy NPs on the surface of g-C3N4. The as-formed Schottky junction accelerated the separation and transfer of photoinduced charge carriers in NiCoAg-CN. NiCoAg-CN exhibited a remarkable photocatalytic hydrogen (H2) yield of 4896 μmol/g, surpassing the performance of NiAg-CN and CoAg-CN by 1.9- and 1.8-fold, respectively. This is attributed to the enhanced catalytically active sites, facilitating rapid charge carrier transfer between the NiCoAg alloy and the g-C3N4 nanosheets. Photoluminescence spectra indicated that NiCoAg NPs served as efficient electron conduits, suppressing charge carrier recombination and promoting direct electron transfer. Alloy NPs synthesized via the proposed strategy efficiently harness solar radiation, facilitate charge carrier transfer, and mitigate recombination, thus advancing efficient photocatalytic H2 generation.