alpha-D-mannosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + 2 GDP-alpha-D-mannose = alpha-D-mannosyl-(1->3)-alpha-D-mannosyl-(1->3)-alpha-D-mannosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + 2 GDP + 2 H(+) — reaction