The pathway in which formaldehyde is used as a carbon source in the ribulose monophosphate cycle. Methanotrophic bacteria produce formaldehyde from the oxidation of methane and methanol, and then assimilate it via the ribulose monophosphate cycle to form intermediates of the central metabolic routes that are subsequently used for biosynthesis of cell material. Three molecules of formaldehyde are assimilated, forming a three-carbon intermediate of central metabolism; in this pathway, all cellular carbon is assimilated at the oxidation level of formaldehyde.