Vivoactive 3 not recording elevation data.

Some (other) issues for another user were resolved by a Master Reset :

Master reset of the vivoactive 3:

- Switch off device (if necessary, press and hold button for 15 seconds).
- Press button briefly to switch on device.
- As soon as the Garmin logo (blue triangle) appears, press and hold the button until the watch vibrates twice. Then release the button.
- Wait for the third vibration and press and hold the button again until the fourth vibration.

If everything was done correctly, you should be able to select the language again.

But ATTENTION: All data will be deleted! Back up with Garmin Express if you still want to.

- Remove your watch from the Garmin Connect App, as well as from the Bluetooth settings of your smartphone: "Remove and add devices in the Garmin Connect App ".
- Uninstall the Connect app again, and then turn off your smartphone completely for a few minutes.
- After you have turned it back on, reinstall the Connect app on your phone and log in with your usual user data
- First, put the vivoactive 3 n pairing mode: activate Bluetooth pairing mode on the vivoactive 3 series and then press "Add device" in the Connect app.
- Enter the six-digit pin code displayed on your watch.
- Follow the further instructions of the Connect app

You can try to see if the sensors are still working by entering test mode :

To access Test Mode:
1. With unit in “off” state, turn the unit on
2. As soon as the display turns on, press and hold the button
3. Wait for the unit to vibrate 3 times and release the button (there will be a long gap between the second
and third vibrations). The unit will then enter test mode.

I have tried to do this but when I press the button as soon a the blue triangle appears, the watch does not vibrate but turns off.  I have tried several times.  I will try removing the app etc.  Thanks.

Can't help you there, just copied what Garmin Support told someone that fixed his bluetooth issue.