Is it just 1 activity missing? Check for the activity on Garmin Connect Web. If you still don't see it, use the charging clip that came with your fenix 3 to plug it into a computer and sync using Garmin Express.
If still no joy, with the charging clip still connected to your computer, look for the activity (fit file) in the folder D:\GARMIN\ACTIVITY. Then grab that file and drop it here to do a manual upload - connect.garmin.com/.../import-data
