I’ve never heard of this device before, so after some googling, I think if figured out how it’s wired with a normal single pole switch. Here is the manual I found. If this isn’t your device, let me know.
It looks like you can make this work by putting the Z-wave relay up where the light box is located. You should be able to replace the single pole switch as shown in the manual and place your 3-way setup in its place. One assumption, the switches you have been using (with the new z-wave relay) are not momentary.
Here is a diagram using an aeotec switch with power on switching ckt. I have an Aeotec in 1 of my light fixtures in a 3 way. I didn’t not switch with power I went with their option to not use power on switch circuit.
I got off track sorry. You can probably use this diagram as I guide to get you going in right direction.
Make sure you know where every wire goes and you land them in the right place. I had to trace mine all out to get it to function. If I can find my personal diagram I used I’ll share. It’s not EXACTLY the same but will be very close.
The relay and the bulb/socket will be wired in the same box. The two wires that go to your existing switches will stay going to your switches, but will need to be rewired as I sketched.