UniProt Summary: Synthesizes the galactose-alpha(1,3)-galactose group on the glycosphingolipid isoglobotrihexosylceramide or isogloboside 3 (iGb3) by catalyzing the transfer of galactose from UDP-Galactose to its acceptor molecule Gal-beta-1,4-Glc-ceramide. Can also catalyze the addition of galactose to iGb3 itself to form polygalactose structures. {ECO:0000305|PubMed:23378701}.