Catalyzes the epimerization of carbon 2' of the side chain of 7,8-dihydroneopterin triphosphate (H2NTP) to form 7,8-dihydromonapterin triphosphate (H2MTP). Is required for tetrahydromonapterin biosynthesis. 7,8-dihydroneopterin 3'-triphosphate = 7,8-dihydromonapterin 3'-triphosphate Homooctamer. Belongs to the DHNA family.