Transfers the sialyl group (N-acetyl-alpha-neuraminyl or NeuAc) from CMP-NeuAc to the non-reducing terminal galactose (Gal) of glycosphingolipids forming gangliosides (important molecules involved in the regulation of multiple cellular processes, including cell proliferation and differentiation, apoptosis, embryogenesis, development, and oncogenesis) (PubMed:16934889, PubMed:9822625). Mainly involved in the biosynthesis of ganglioside GM3 but can also use different glycolipids as substrate acceptors such as D- galactosylceramide (GalCer), asialo-GM2 (GA2) and asialo-GM1 (GA1), although less preferentially than beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)- Cer (LacCer) (PubMed:16934889). {ECO:0000269|PubMed:16934889, ECO:0000269|PubMed:9822625}. This is the function of ST3GAL5 (ST3 beta-galactoside alpha-2,3-sialyltransferase 5, ENSG00000115525).