You need to first figure out what each prong does. If one is power, then the other should be a completed cicuit when not pressed and the third is circuit completer when pushed. If it is this then it will be simple to hook up. Does the switch have a light in it?
If does not have a light, then you want to hook the Fiero orange wire (at the original switch) to the power prong, then the grey with black strip (at the original switch) to the output prong when the button is pressed. The third prong does not need to be used unless it is an illuminated switch, then that third one might be a secondary power for the light OR a ground, in which case it may need a different hook up.