As I recall, the third button is largely superfluous for most games because nobody wanted to create software that was incompatible with the stock controller. I'm troubled that NCS felt it necessary to specify that PC-Engine pads use a different connector than the western TurboGrafx, as if the people who would be buying this thing didn't already know that.
Apropos of nothing, I haven't fired up any of my PCE or Turbo hardware in a while. I own one of every American iteration of the hardware, plus a lovely white Duo-R unit that I repaired with the information gleaned from GamesX. They're all plugged in and ready to go, but I don't have an uninterrupted stretch of time to play through Dungeon Explorer like I used to years ago.