I figured it out! My first Vivocactive 3 would not pair with my phone and I was advised by Garmin to exchange it at the store. I set up the second device, and after reading a lot of different advice here, I had gotten myself in to a situation where Garmin Express and the Garmin Connect App each had one device (the new one) Garmin.com had zero devices registered, and Garmin Connect had both the old and the new.
Then in Garmin Connect (the desktop version) I stumbled on the cog icon in the upper right-hand corner. (See image below, which shows the cog menu on click). Here's what I did:
I don't know how to get my device registered again in Garmin.com, and I don't know that I care. I just know that Garmin Connect desktop, Garmin Connect app, and my device are all talking to each other now with no extra devices hanging around.
Thank you! This all worked well for me. I had my Garmin Z82 golf range finder "stuck" in the android connect app. By using the link, and then going to the online (web) version of the connect app, I was able to fully remove the device, and then follow the original instructions to add. Fore!
Thanks for the help! Well done!