Catalysis of the reaction: a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + CMP + H+.
N-acetyllactosaminide alpha-2,3-sialyltransferase