I believe it is possible to load the Tesla 3 key card into the Apple Watch wallet and start using the Apple Watch to open the Tesla 3 (without additional app). Am I correct? Do you know how? Thank you and very much appreciated (I have googled, read a lot but nothing clear)
Not directly no. Tesla does not support apple watch, does not have an apple watch app, and the phone as key feature does not work through an apple watch.
The reason nothing is "clear" when you read it, is because third party apps can also trigger the door open, start car abilities the same as the tesla APP (which, is NOT the same as "phone as key).
The basic difference is, starting the car or opening the door from the app requires the car have access to a network connection, either its own cellular connection or wifi. If its in a parking garage with no cellular / wifi, you can not open the door using the APP to open it.
In those circumstances, however, "phone as key" will work, because that specific feature uses Bluetooth LE (low emissions / energy bluetooth), and that does NOT require a network connection, nor does it work with apple watch.