The third button is a blank from the factory. Bolton Hooks is developing a kit to add functionality to this blank switch. It is a pricey upgrade, but it you have to replace or add the handle switch; it may be an option.
https://www.boltonhooks.com/in-the-works/
I love mine. It needed some tweaking as I got it before they started adding a kickstand and making it quick hitch compatible. It is currently set up for snow removal.