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
1. EC 2.4.99.42. MetaCyc 2.4.99.4-RXN3. Reactome: R-HSA-1022129 "ST3GAL4 transfers Neu5Ac to terminal Gal of N-glycans"
4. Reactome: R-HSA-1912378 "Sialylation of Pre-NOTCH"
5. Reactome: R-HSA-2046285 "The keratan chain can be capped by N-acetylneuraminic acid"
6. Reactome: R-HSA-3656258 "Defective ST3GAL3 does not transfer SA to keratan"
7. Reactome: R-HSA-4084984 "ST3GAL1-6 transfer Neu5Ac to terminal Gal (alpha-2,3 link)"
8. Reactome: R-HSA-9603987 "ST3GAL3 transfers Neu5Ac to Type 1 chain to form Type 1 MSGG"
9. Reactome: R-HSA-9605600 "ST3GAL3,4,6 transfer Neu5Ac to Type 2 chain to form Type 2 MSGG"
10. Reactome: R-HSA-981497 "ST3GAL1-4 can add a sialic acid to the T antigen at the alpha 3 position"
11. RHEA: 21616