There are a few JAMMA era Japanese shooters that use a 3rd button for "Bomb" or some other function. I put 3 on my vert MAME cab to be safe, but third button rarely gets used.
If this is going to be MAME, I'd put it on there just to cover all the games.
Japanese Version of Ikaruga IIRC. Its supposed to be superior to the american version. Some say its the best shooter ever. But I suspect thats being too generous. But 97% or more use 2 buttons or less.