Dol-P-Man:Man(5)GlcNAc(2)-PP-Dol alpha-1,3- mannosyltransferase that operates in the biosynthetic pathway of dolichol-linked oligosaccharides, the glycan precursors employed in protein asparagine (N)-glycosylation. The assembly of dolichol-linked oligosaccharides begins on the cytosolic side of the endoplasmic reticulum membrane and finishes in its lumen. The sequential addition of sugars to dolichol pyrophosphate produces dolichol-linked oligosaccharides containing fourteen sugars, including two GlcNAcs, nine mannoses and three glucoses. Once assembled, the oligosaccharide is transferred from the lipid to nascent proteins by oligosaccharyltransferases. In the lumen of the endoplasmic reticulum, adds the first dolichyl beta-D-mannosyl phosphate derived mannose in an alpha-1,3 linkage to Man(5)GlcNAc(2)-PP-dolichol to produce Man(6)GlcNAc(2)-PP-dolichol (PubMed:10581255). Man(6)GlcNAc(2)-PP- dolichol is a substrate for ALG9, the following enzyme in the biosynthetic pathway (PubMed:10581255). . This is the function of ALG3 (ALG3 alpha-1,3- mannosyltransferase, ENSG00000214160).