Q: What LEDs are Illuminated when the R-3W is Installed on a 120V Single Phase System?

A: When the R-3W or other Voltage Indicating PESD is wired into a 120VAC (or 220-250 VAC) Single 2W + GND System, all wires that are connected will have their corresponding LEDs illuminated.  This is because there is no balance between phase to phase voltages, as is seen with 480VAC 3 Phase Systems and 240V US Split-Phase Systems.    

There is more than one way to wire it and customers have chosen to wire it in different ways in the field, so you may not have all LEDs illuminated as would be the case as wired per the below since every wire is landed to Hot, Neutral, and Ground.  If a wire is not connected, it will not illuminate.