FBSOpenApplicationErrorDomain error 3 when trying to launch a watch kit app on a physical device

This was a permissions issue on the Apple Watch.

To fix:

1. Launch the app once manually (not through Xcode).

   You will see the following message:

   Untrusted App Developer
   Do you trust the developer "..." to run apps on your Apple Watch?
   [Trust]
   [Don't Trust]

2. Tap Trust.

3. Now you can run the app through Xcode with no issues.

Had same issue:

1. Go to settings on iphone
2. General > Device management
3. choose your developer app from list
4. click on trust