Catalysis of the reaction: 3-O-N-acetyl-D-galactosalaminyl- + UDP-N-acetyl-D-glucosamine = 3-O-N-acetyl-beta-D-glucosaminyl-1,3-N-acetyl-D-galactosaminyl- + UDP. Linkage of the glycan to the protein occurs via the oxygen atom in the side chain of an L-serine or L-threonine residue.
mucin core 3 beta3-GlcNAc-transferase activity