MAME button combo setup?

I would like to know how to make a key count as multiple buttons at one in MAME, if that's possible. I've been playing a ROM of Asura Blade a lot recently, and would like to set up a key for buttons a+b+c so as to be able to throw my weapon. Otherwise, it's near impossible due to keyboard overloading (3 keys at once). The only way to do this without the keyboard overloading, is to hit all 3 at the EXACT same time. A split second off, and it won't work. so, how can I set up such a key? I don't find the help section in mame helpful for anything, so telling me what topic it's in won't help. If I can't do this... ah, well. It'll just be a lot harder to beat Goat with Zam b.