<p>Trimetallic nanoparticles have shown significant potential as multifunctional nanomaterials due to their superior photocatalytic performance, making them highly promising in materials science and nanotechnology. In this study, we designed and synthesized a trimetallic nanostructure consisting of a gold nanorod (Au NR) core, a silver (Ag) intermediate shell, and a silver-platinum (AgPt) alloy outer layer. The Au@Ag core–shell structure was first prepared by depositing silver onto the Au NR surface through a reduction reaction. Subsequently, an AgPt alloy was deposited via a reduction and replacement process, followed by oxidative etching to remove excess silver, resulting in the etched trimetallic nanostructure (Au@AgPt-E). Using methylene blue (MB) as the model system, the photocatalytic activity of Au@AgPt-E under visible and near-infrared light irradiation was significantly enhanced, achieving a rate 4.19 times higher than Au@Ag@AgPt and 7.67 times higher than single-metal Au NRs. The performance enhancement is primarily attributed to the surface plasmon resonance (SPR) effect generated by the gold nanorod core in Au@AgPt-E. The SPR effect significantly enhances the light absorption of the gold nanorods, especially within specific wavelength ranges, allowing for efficient conversion of light energy into heat. This energy conversion process not only improves thermal efficiency but also facilitates the generation and separation of charge carriers, thereby increasing the efficiency of the catalytic reaction. Additionally, the Au@AgPt-E undergoes an etching process, which increases the number of catalytic active sites on the AgPt shell, further enhancing the catalytic performance. The combined effects of these factors result in a significant improvement in the material’s performance. This study provides a novel approach for developing efficient plasmon-mediated photocatalysts and demonstrates broad application prospects in photocatalytic reduction processes.</p>