The flyer I got for this group shows four people, as does a website I saw with pictures of them performing (in Washington DC), but there were only two members of the band on the evening of February 23rd, 2010.
Incidentally, the spelling of their name (based on ミツマッシュ) as "Mitsumassyu" is using a defunct and outdated system of "romaji" for writing Japanese with English letters. They should be spelling their name: "Mitsumasshu", unless, of course, they are doing it intentionally to have a quirky name.
新宿駅西口あたり ミツマッシュのライブ 東京
Lyle (Hiroshi) Saxon