I moved to the Mate 20 Pro from Samsung S8+ and had issues connecting and syncing my Gear S3 as it's so much better than the Huawei watch.
Tried resetting the watch several times before finding the solution.
Make sure you have the Wearable and gear S plugin installed. Delete the watch from the phone, reset the watch and run the Wearable app. Don't connect the watch via Bluetooth first, let the app do that. Run the setup. All works fine now.