You're going to be playing hit and miss to find Qi chargers that work with the Apple Watch 3. By all accounts the AW3 isn't 100% Qi compatible - Apple did state that they shared information with the Qi consortium, so possibly there will be an update to the spec that the AW3 will be 100% compatible with.
All we know is that it will work with Apple's upcoming AirPower mat.
We should likely see in time some of the Qi charging mats that do support multiple devices will have their mats testing and certified to support the AW3, but until then, you're just going to have to try a bunch out to find one that will work for you.