The incorporation of hybrid nanomaterials and fiber architectures into fiber-optic biosensors has proven to be effective in improving sensing performance. This study developed a plasmonic biosensor using a tapered multimode fiber (MMF)–multicore fiber (MCF)–MMF structure functionalized with a nanocomposite of gold nanoparticles (AuNPs), tungsten disulfide quantum dots (WS₂-QDs), and multi-walled carbon nanotubes (MWCNTs) for selective triclosan detection. Combining WS₂-QDs and MWCNTs with AuNPs leverages their individual advantages, creating an active sensing interface with abundant analyte-binding sites, thereby substantially improving responsiveness. The composite-modified structure also promotes higher-order mode generation, which is critical for sensitivity enhancement. When optically excited, the periodic plasmonic geometry produces strong evanescent fields that intensify interactions between the immobilized nanocomposite and target molecules. Transmission spectroscopy demonstrated that the biosensor achieved a detection limit of 3.87 µM within the 0–40 µM range. These results show that the strategic combination of WS₂-QDs and MWCNTs with AuNPs significantly amplifies localized surface plasmon resonance (LSPR) effects. This architecture offers a promising approach for developing sensitive fiber-optic biosensors, particularly for environmental monitoring of agents like triclosan.