a 3-O-{beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->6)-[beta-D-Gal-(1->3)]-alpha-D-GalNAc}-L-threonyl-[protein] + CMP-N-acetyl-beta-neuraminate = a 3-O-{alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->6)-[beta-D-Gal-(1->3)]-alpha-D-GalNAc}-L-threonyl-[protein] + CMP + H(+)