welcome to the joys of usb, Mame and lightguns.
1. Can I suggest you install normal mame to get this working first? Its much better known and there are heaps of threads in this forum to help you.
2. regardless it sounds like windows and mame are recognising your gun as Gun3. This is probably because two other mice device are present. Presuming we are talking about your laptop here your onboard mouse is mouse 1- ie Gun1. If you have an external mouse in as well then its Mouse 2 - yep Gun 2. So your next available mouse (Gun) is Gun3.
You could just change the x and Y axis to Gun3 and it will work. see
http://www.act-labs.com/LightGunSetUp/LightGunSetUp.htm as a guide but you'd have to do this for each game (yes I know its for Act labs but the principles are the same).
But when it comes to your dedicated PC for the cab IF you have no other mouse device needed - remove the mouse before setting up the gun(s) and the guns will then be gun1 and Gun 2
If you do need to have another mouse device then you need a cfg file for the controllers. Not sure how thats done in Mame32- but can probably find out