beta-galactoside (CMP) alpha-2,3-sialyltransferase activity https://github.com/geneontology/go-ontology/issues/30193 Reactome:R-HSA-9605600 Reactome:R-HSA-9603987 RHEA:56204 Reactome:R-HSA-9845538 RHEA:56208 Reactome:R-HSA-9844860 Reactome:R-HSA-1022129 Catalysis of the reaction: CMP-N-acetylneuraminate + beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl-R = CMP + alpha-N-acetylneuraminyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl-R. Reactome:R-HSA-2046285 Reactome:R-HSA-3656258 RHEA:21616 Reactome:R-HSA-4084984 EC:2.4.3.4 GO:0003836 Reactome:R-HSA-1912378 molecular_function MetaCyc:2.4.99.4-RXN sialyltransferase activity