I have a Juntek battery monitor and mine is set for two wire operation as the shunt is supplied from the same battery that supplies power to our caravan through the shunt. 3 wire operation is for when the shunt is supplied from a separate battery supply from the above mentioned battery. A red light on the shunt means it's working and should be sending data to the display unit.
2011 Southern Star 630 caravan, 300 w solar, 2x 100 Ahr LifePo4 batteries, 1700w inverter, 160 ltr of fresh and grey water. Tow tug: 2016 Ford Ranger 3.2 TD 4x4 auto.