How do I prevent an Android device to connect to my Fenix 3 watch?

Because it is already paired with the phone.

Because it is already paired with the phone.

And what's the problem? You can have the watch paired with multiple devices.

Look for example here: pairing watch to 2 devices (phone and tablet)

That's not exactly true. you can only have the watch paired to one device at any one time. What is happening in that post is that the user changing the pairing between two devices by repairing each time. This can also change the setup in the watch based on whatever configuration that GCM on each device has for the watch that isn't stored centrally in your Garmin account. 

I have no desire to do this & I don't want to sync my watch to the 2nd device or have to keep changing the pairing back & forward. I only want to keep my watch paired to the phone but to be able have GCM on the tablet without it trying to pair with my watch whenever it's in BT range. GCM should only instigate a pairing if I request it not automatically without my permission 

hotdogpartysausage said:

What is happening in that post is that the user changing the pairing between two devices by repairing each time.

No, he is not. Ask seilogramp, he is one of the moderators here, and a long time Garmin user. He knows what he is doing, and would not advise some BS.

Spot on @hotdogpartysausage, that is how I realized what was happening. I have the language on my tablet different from the one on my phone so whenever the tablet connects to my watch it will reset the language on my watch. I cannot understand that watch connection details are synchronized but not watch/device settings.

Anyway it seems that this is a GCM issue. Some will call it a defect and others a feature. For me at least this is one reason I will look for other brands whenever I feel the need for a new watch.

Have you tried it? Because when I did it would pair & connect to my tablet but not back to my phone without re pairing.

Even if it did work you are missing the point. Well for me anyway. I have no desire to connect my watch to another device when I'm quite happy with it only being paired & connected to my phone for the reasons explained above. So I don't see why GCM should try to force you down that route 

hotdogpartysausage said:

Have you tried it?

I do not use any tablet, but if you read the thread I linked previously, you see seilogramp does, as well as other users on this forum. You may be able to find more similar threads - for example pshare uses both, a phone and a tablet with his watch - he speaks about it in this thread, using the same method (another member at the end of the thread confirms it too). Pshare apparently have it paired with 3 devices in the same time, as he wrote in another thread.

Seilogramp tells:

"With GCM installed on each device, I turn off BT on my phone if I want to use the iPad, then reverse it when done."

Hence no re-pairing, just turning the BT off or on, when needed.

trux said:

Hence no re-pairing, just turning the BT off or on, when needed.

Not a solution nor suitable workaround. I have bluetooth turned on all the time to have both devices connected to headset. This is possible with Jabra devices but not Garmin watches it seems.

If you really need to listen both devices simultaneously on the headphones, in the same time while you are using Garmin Connect, then instead of turning BT off on the entire device, you can use Bluetooth Firewall or a similar app to turn off BT selectively just for Garmin Connect Mobile, on one of the devices.