@bmtanner +1 on above post - and also to emphasize again that you must install at the incoming Line end, not the load end I created this diagram for another member - hopefully that is helpful for you...

@Senior_Vagabond it really depends on how deep your box is - some responders might have no issue, but may not be relevant to your situation unless their switch box is the same. I have two Kasa...

Ferinthul wrote Voltage is on both travellers for the HS210... @Ferinthul Was this with them disconnected or connected? You can't really have voltage on both travelers - the way the switch works,...

@Ferinthul did you remove the wire from the switch to test it? It is possible that you can voltage back-feeding through the switch from one of the travelers Important to disconnect and validate that...

Ferinthul wrote That will be the gang where i will install the 3-way... and i leave the "dummy" switch at the other gang. Well they are both 3-way - but I'm sure you mean the HS210 Smart Switch...

@Ferinthul OK, that is helpful - you definitely have a Neutral System incorporated so that is a positive. The Romex that contains the Black/Red/White/Ground - is the Traveler Romex - as an idividual...