Specifically glucosylates the 3'-hydroxy group of delphinidin 3,5-di-O-glucoside to produce gentiodelphin. Shows a strict specificity for UDP-glucose as donor. delphinidin 3,5-bis-O-beta-D-glucoside + UDP-alpha-D-glucose = delphinidin 3,3',5-tri-O-beta-D-glucoside + H(+) + UDP Abundant in petals and barely detected in leaves. Expressed early in petals development and decreases in fully opened flowers. Significantly inhibited by Al(3+) and Co(2+), moderately by Zn(2+) and not by Mg(2+), Ca(2+), Mn(2+), EDAT or EGTA. Competitively inhibited by UDP. The N-terminus is blocked. Belongs to the UDP-glycosyltransferase family.