GO:0047273:   galactosylgalactosylglucosylceramide beta-D-acetylgalactosaminyltransferase activity
Catalysis of the reaction: UDP-N-acetyl-D-galactosamine + alpha-D-galactosyl-(1->4)-beta-D-galactosyl-(1->4)-beta-D-glucosylceramide = UDP + beta-N-acetyl-D-galactosaminyl-(1->3)-alpha-D-galactosyl-(1->4)-beta-D-galactosyl-(1->4)-beta-D-glucosylceramide.