Catalysis of the transfer of a mannose residue to an oligosaccharide, forming an alpha-(1->3) linkage
1. GOC: mcc
2. PMID 105215413. Reactome: R-HSA-446188 "ALG3 transfers Man to N-glycan precursor (GlcNAc)2 (Man)5 (PP-Dol)1"
4. Reactome: R-HSA-446208 "Addition of a second mannose to the N-glycan precursor by ALG2"
5. Reactome: R-HSA-4549368 "Defective ALG2 does not transfer a second Man to N-glycan precursor"
6. Reactome: R-HSA-4720473 "Defective ALG3 does not add mannose to the N-glycan precursor"