2 GDP-mannose + alpha-D-Mannosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-diphospho-ditrans,octacis-undecaprenol <=> 2 GDP + alpha-D-Mannosyl-(1->3)-alpha-D-mannosyl-(1->3)-alpha-D-mannosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-diphospho-ditrans,octacis-undecaprenol