I have used Basecamp now after reading about it there. It did infact convert gpx files to FIT and download to the Fenix 3. My primary use had been to create maps via mapmyride etc. and then send to the Garmin 500. To do that required exporting from mapmyride as TCX files, then using gpsies to convert from TCX to GPX and finally downloading to the Garmin Edge 500. When I first got the Fenix 3 I’d assumed I’d stop using the Edge 500 and use the Fenix instead.
What I’ve actualy done is to stop using mapmyride, and use Garmin Connect to make routes instead. Making the routes/maps on there is as easy, possibly easier than mapmyride and also eliminates all the conversion steps. Once you have a route created on Garmin Connect, you can simply send them directly to the (docked) Garmin device. For cycling I still send them to the edge 500 and use that in map mode on the bike handlebars. I use the Fenix 3 as a traditional sports watch, HRM etc. to record what I actually do.