The GE Cync 3-Wire Smart Switch brings support for voice commands to traditional lighting fixtures and doesn't require a neutral wire, making it a good fit for homes with aging electrical systems.

One wire comes in from the power source, and one wire goes out to the light (s). A 3-way switch adds two terminals for travelers, which connect two 3-way switches together.

In the diagram on the link they are using a three conductor cable after the first three way switch. Black, Red, and White with a separate ground.