Parameter 7 has the answer. " Parameter 7: Choose the type of wall switch connected to the relay and it’s on/off behavior.
Values: 0 – toggle switch (device changes status when switch changes status); 1 – momentary switch (one-button switch like the ZAC99); 2 – toggle switch with fixed actions (always toggle up for on, always toggle down for off); 3 – 3-way impulse control (for selected 3-way scenarios, refer to the notes in the wiring instructions for your set-up); 4 – garage door mode (simulates a momentary push button switch in the Z-Wave interface, used for garage door opener applications). Default: 2.
Size: 1 byte dec"
If you set this to "0" it will change the state of the relay whenever the any switch in the 3-way is changed. This works great. Now for the bad news. This works great in my "test setup" with the two 3-ways mounted in a single switch plate, 2 inches apart. In real life, there is an issue with the inductive voltage often found in 3-way circuits. Much of my home is wired like yours with switch loops with the neutral only at the fixture. The ZEN 51 seems to need 0 volts on the line for it to function using the existing switches. I was showing less than a volt on my cheapo meter and it was enough to hold the switch in its current state. I could remove and touch the .3 volts wire from the ZEN and it switched back and forth. The ZEN 51 always works when controlled from HE but won’t work with the existing switches. I did get it to work with a single switch that had the neutral in the box. There are a lot of uses for the ZEN if controlled by HE but in its current state I couldn’t use it for what I was trying to do. I had some brief back and forth with Zooz support but I put this one in the box to try again later. There was another post Zooz Zen51 and Zen52 - double relay & dry contact with some more info.
