I've only had my Vivoactive 3 for a few weeks so am still trying to figure out a few things. In the beginning, when I did an indoor rowing session, I could go into the Garmin Connect app on my mobile, edit the activity and add in the distance the indoor rower said.
However, that functionality is now greyed out. When I go into the activity on my phone, the "distance" option is greyed out and you can't edit it. I can go into Garmin connect on my desktop and edit it there so I'm unsure why this has been removed as a function on the phone, it's a bit tedious?