Catalysis of the reaction: a 1,2-diacyl-sn-glycerol + UDP-alpha-D-glucose = a 1,2-diacyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerol + H+ + UDP.
UDPglucose:1,2-diacylglycerol 3-D-glucosyltransferase activity