Catalysis of the reaction: beta-D-galactosyl-(1->3)-N-acetyl-beta-D-galactosaminyl derivative + CMP-N-acetyl-beta-neuraminate = N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-beta-D-galactosaminyl derivative + CMP + H+.
monosialoganglioside sialyltransferase