Modifying strange 3 way switch wiring
With only 3 wires plus ground you don't have a neutral on an existing 3 way switch. You have a hot. I was taught on switch legs to run the white as the always hot. Then you have 2 travelers until recent code changes this was the red and black and the ground bare / green wire.  I have seen so many different ways to wire 3 ways over the years many that were code violations it would be hard to describe them all but for a mechanical spdt switch combination to work the 3 wires are required, just because you have a white wire don't assume it is a neutral especially with a home that is 20+ years old. If you don't have access to the wiring how can you rewire it would be my concern.