I'm having the exact same issue. TestFlight 3.3.0 won't install my app on the Apple Watch Series 3, it crashes attempting to install. However, TestFlight is installing the same build on my Apple Watch SE without issues at all. Too bad because there's no way to install apps on the Apple Watch other than using TestFlight (or Xcode), as far as I'm aware...

Anyone had the chance to test this issue in watchOS 8.7.1 to see if it still happens? In watchOS 9.0 is fixed.