When trying to set up my Fenix 3 to connect to my iPhone wifi hotspot, the iPhone didn't appear on the list of available wifi networks in Garmin Express; despite the laptop running Garmin Express being successfully tethered by wifi to the iPhone. Nothing I did at home would fix this, no matter how many times I refreshed the list of available wifi networks. I then tried again with the same laptop+GE, but at work, and they found the iPhone hotspot straight away. Eventually, I worked out what was going on. When setting up the Fenix 3 wifi, Garmin Express can only list a small number of the available wifi networks (the list of available networks shown on the laptop at home was much longer the the list in Garmin Express). At home there are so many networks to display (my neighbours' home wifi networks etc.) that the iPhone hotspot never made it on to the list; no matter how many times I refreshed the list. But at work, there's only 2 other wifi networks to list, so no bother.
Essentially, if you're having trouble finding your wifi network when setting up a Fenix 3 (and other wifi capable Garmin devices) in Garmin Express, it may be there's too much wifi clutter from your neighbours. With a mobile phone hotspot and laptop you can just move somewhere with less clutter, as I did by trying again at work. But if you're struggling to find your own home fixed wifi in Garmin Express on a desktop PC, I'm not sure what the solution is beyond shielding your computer from other wifi signals using some tin foil or visiting all of your neighbours and asking them to switch all their routers off temporarily! But really the fix should come from the Garmin Express software in due course being configured to list all of the available wifi networks; over to you Garmin.