This Nikudan Sanyushi medal commemorates the 1932 Shanghai Incident. The "three human bombs" were supposedly three soldiers that had loaded themselves up with explosives and attacked a chinese position. In reality, an officer had forced a report to bolster Japanese moral and bring honor to his unit. The three soldiers mentioned in the report had actually died in a bayonet charge. The story took a life of its own and songs, medals, monuments, and other mementos were created to commemorate the "three human bombs". I can't seem to find any info on whether this particular medal is government or private issue, but I have seen others of different design that were private issue.