Catalysis of the reaction: 4-beta-D-galactosyl-O-beta-D-xylosylprotein + UDP-galactose = 3-beta-D-galactosyl-4-beta-D-galactosyl-O-beta-D-xylosylprotein + UDP.
UDPgalactose:4-beta-D-galactosyl-O-beta-D-xylosylprotein 3-beta-D-galactosyltransferase activity