Catalysis of the reaction: N-acetyl-beta-D-galactosaminyl-(1,4)-beta-D-glucuronosyl-proteoglycan + UDP-alpha-D-glucuronate = beta-D-glucuronosyl-(1,3)-N-acetyl-beta-D-galactosaminyl-(1,4)-beta-D-glucuronosyl-proteoglycan + UDP.
N-acetylgalactosaminyl-proteoglycan 3-b-glucuronosyltransferase activity