Como encontrar mi reloj Fénix 3 si lo perdi?

That would be only possible if the watch was within 10 meters of your phone, since the watch does not have any possbility to communicate with the Net without a working Bluetooth connection to the phone. 

Also check this article: https://support.garmin.com/en-IE/?faq=jp2Q6RidNz2gT8ND3TahH9