The catalytic performance of Mo-based transition-metal-dichalcogenide (TMD) monolayers is intrinsically tied to their physicochemical properties. However, the limited chemical diversity among these materials constrains their versatility for key catalytic processes, including carbon dioxide (CRR), nitrogen (NRR), and oxygen (ORR) reduction reactions. This study employs density functional theory (DFT) calculations to investigate the impact of chalcogen vacancies on the properties of MoS2, MoSe2, and MoTe2, focusing on the adsorption behaviors of CO, NO, and NO2. The findings reveal that chalcogen vacancies not only enhance surface reactivity but also impart distinctive physicochemical characteristics to each TMD. These effects arise from intrinsic bonding differences, resulting in distinct charge availability at exposed Mo atoms and variations in vacancy dimensions, which shape specific surface interactions. Hence, while adsorption differences between pristine surfaces are generally negligible for catalysis, vacancies amplify them by over an order of magnitude, resulting in pronounced material-specific behaviors. Moreover, varying vacancy dimensions affect how species incorporate into defects, further enhancing the differences. These characteristics unlock substantial potential of TMD sheets for distinct surface chemistries, transforming them from relatively similar to markedly different as defect density rises. Consequently, our findings provide fundamental insights for tailoring these materials toward applications in electro- and photocatalysis.