If you've just purchased a Sony SmartWatch 3 you will notice on first boot that an update to the watch immediately begins...
You will then also notice that update keeps failing and the watch keeps restarting this downloading update loop.
To fix:
Unpair your SW3.
Hold power button down on SW3 and scroll to "reset watch"
Uninstall Android wear.
Install an earlier version of Android Wear, this one will do
http://www.apkmirror.com/apk/google-inc/android-wear/android-wear-1-0-5-1724356-android-apk-download...Install it and run it.
Pair as normal and setup watch again.
You will now be able to update the watch and finalise installation.
The reason...
Latest android wear will disconnect from Bluetooth unless an active watch is connected, as the watch is updating, it's not fully active yet.