Background: Pathogenic ST3GAL3 variants cause neurological and cognitive impairment, defining a distinct congenital disorder of glycosylation (ST3GAL3-CDG). Nonetheless, limited enzyme characterization exists due to the lack of a non-radiochemical assay. Methods: Here, we developed an LC-MS/MS-based method using the artificial substrate para-nitrophenyl-lacto-N-biose (LNB-pNP; Galβ1,3GlcNAcβ1-O-C6H4NO2) to measure ST3GAL3 activity in vitro. Results: A peak corresponding to sialyl-LNB-pNP was detected in reactions with homogenate from HEK-293T cells transfected with pCDNA3 ST3GAL3 plasmid, but was virtually absent in mock-transfected cells. A substrate dependence curve provided an apparent Km value for the substrate (0.40 mM) and closely matched values from prior radiochemical methods. No activity was detected with homogenates from cells expressing pathogenic ST3GAL3 variants, except p.A13D, which is known to retain about 10% of residual activity. Compared to ST3GAL4 and ST3GAL6, ST3GAL3 showed markedly higher specificity toward LNB-pNP, lactotetraosylceramide (Lc4) and asialo-GM1, which are rather specific substrates. Instead, neo-lactotetraosylceramide (neoLc4) was processed by all three ST3GALs. Conclusions: These findings suggest that ST3GAL4 or ST3GAL6 cannot compensate for ST3GAL3 loss in the biosynthesis of gangliosides sialyl-Lc4 and GM1b, but may do so for sialyl-neoLc4. This non-radiochemical assay enables screening and diagnostic evaluation of novel ST3GAL3 variants potentially associated with ST3GAL3-CDG.