Catalyzes the reaction: UDP-alpha-D-xylose + -3-O-(beta-D-glucosyl)-L-serine = UDP + -3-O-(alpha-D-xylosyl-(1->3)-beta-D-glucosyl)-L-serine.
UDP-D-xylose:beta-D-glucoside alpha-1,3-D-xylosyltransferase activity