Adding ammeter as the 3rd is a no brainer. It's on my 'todo' list, along with building a REAL grille guard.
A voltmeter is a low buck 'cheat' and I've tossed them out of the rest of the tractors and wired in real ammeters. A voltmeter only shows the 'surface voltage' of battery or alternator output NOT the actual flow of electrons ( current) going to/fro the battery. It's in the same catagory as TPMS...