Required for N-linked oligosaccharide assembly. Adds the sixth mannose residue in an alpha-1,3 linkage onto the dolichol-PP-oligosaccharide precursor dolichol-PP-Man(5)GlcNAc(2). a dolichyl beta-D-mannosyl phosphate + alpha-D-Man-(1->2)-alpha-D-Man-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-alpha-D-GlcNAc-diphosphodolichol = a dolichyl phosphate + alpha-D-Man-(1->2)-alpha-D-Man-(1->2)-alpha-D-Man-(1->3)-[alpha-D-Man-(1->3)-alpha-D-Man-(1->6)]-beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-alpha-D-GlcNAc-diphosphodolichol + H(+) Protein modification; protein glycosylation. No obvious phenotype under normal and high temperature or salt/osmotic stress conditions. In the absence of ALG3 activity, the N-glycans transferred to proteins are aberrant, indicating that the oligosaccharyltransferase (OST) complex is substrate-tolerant. Belongs to the glycosyltransferase 58 family.