I can upload basic metrics from my Fenix 3 (steps, HR, etc) and see them in the app but I can't upload an activity I did earlier today (a run) to see the stats on it.
You can upload the activity via PC: the FIT files are directly accessible via USB mass storage in the "Activity" directory.
This allows you to upload FIT files to cloud-based services such as Strava to see your running stats, or into a PC-based application to do the same locally.