Glycosyltransferase that transfers a galactosyl group from UDP-galactose to soyasapogenol B monoglucuronide in the biosynthetic pathway for soyasaponins. soyasapogenol B 3-O-beta-D-glucuronate + UDP-alpha-D-galactose = H(+) + soyasaponin III + UDP Belongs to the UDP-glycosyltransferase family.