The function of GAL3ST3 (galactose-3-O-sulfotransferase 3, Ensembl gene identifier ENSG00000175229) is as follows. Catalyzes the transfer of a sulfate group from 3'- phosphoadenylyl sulfate (PAPS) to the C-3 hydroxyl group of terminal non-reducing beta-1,4-linked galactose residues in both N-glycans and O-glycans (PubMed:11323440, PubMed:11356829, PubMed:14701868). It acts efficiently on type 2 chains (Gal-beta-1,4-GlcNAc-R) found in N-glycans and core 2-branched O-glycans (Gal-beta-1,4-GlcNAc-beta-1,6(Gal- beta-1,3)GalNAc-R) (PubMed:11323440, PubMed:11356829, PubMed:14701868). In contrast, it shows poor activity on type 1 chains (Gal-beta-1,3- GlcNAc-R) and intermediate activity on core 1 structures (Gal-beta-1,3- GalNAc-R) (PubMed:11323440, PubMed:11356829). Participates in the glycoconjugates sulfation namely the 3'-sulfated Lewis X epitope (Gal- beta-1,4(Fuc-alpha-1,3)GlcNAc), by sulfating N-acetyllactosamine (Gal- beta-1,4-GlcNAc) at the 3' position; then the resulting modification is fucosylated by FUT5 (PubMed:11356829). {ECO:0000269|PubMed:11323440, ECO:0000269|PubMed:11356829, ECO:0000269|PubMed:14701868}.