Involved in triacylglycerol (TAG) biosynthesis (PubMed:22760209, PubMed:30467384). Catalyzes the acylation of the sn-3 hydroxy group of sn-1,2-diacylglycerol using acyl-CoA (PubMed:22760209, PubMed:30467384). May preferentially use linolenoyl-CoA as substrate and to a lesser extent linoleoyl-CoA (PubMed:22760209). May contribute to the active recycling of linoleate and linolenate into TAG when seed oil breakdown is blocked (PubMed:22760209). a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Expressed in germinating seeds 48 hours after exposure to the light, just after the emergence of the radicle. No visible phenotype under normal growth conditions. Belongs to the diacylglycerol acyltransferase family. The subcellular localization of a truncated sequence of DGAT3, lacking AA 1-75, has been shown to be cytosolic (PubMed:22760209). However, the N-terminus of the full-length protein is predicted to contain a transit peptide for chloroplast targeting. Therefore, the localization of DGAT3 is unsure.