Don't use Iphone + Strava. Connect GarminConnect directly to your Strava?
I normally allways wear my 920XT, so I will allways use it on my runs. And it transfers everything directly to Strava.
I would guess that the Iphone has much worse distance calculation then your Fenix 3.
https://support.strava.com/hc/en-us/articles/216918057-Uploading-from-a-Garmin-device-#automatic
I use my phone to upload the activites, so I have connected the Strava app and the Garmin app together.
Works great!
No, it's exactly the opposite. My Iphone 6 is spot on with gps accuracy while the Fenix 3 always reads a lower distance. At a 10K training which was verified by my on a sattelite map with the course created by myself, the Iphone 6 showed exactly the same distance as what I've created, while the Fenix 3 said 9.77K which is 230m lower and a much worse pace !