GO:0008118:   N-acetyllactosaminide alpha-2,3-sialyltransferase activity
Catalysis of the reaction: CMP-N-acetylneuraminate + beta-D-galactosyl-1,4-N-acetyl-D-glucosaminyl-glycoprotein = CMP + alpha-N-acetylneuraminyl-2,3-beta-D-galactosyl-1,4-N-acetyl-D-glucosaminyl-glycoprotein.