I know that both the front and rear track of the Model 3 is about 2" wider than my Prius. Are you saying the wheels on the Model 3 are tucked in way more than the Prius?
The track of a car is defined as the width between the centerline of the wheels.
Model 3 Track: 62.2"
Prius Prime Track: 60.6" (rear, which is wider than the front)
Altima SR Track: 62.0"
Now, I currently have a 2017 Nissan Altima in my garage, which appears to be similar to the Model 3 in dimensions, though surprisingly about 6" shorter with a length/width/height of 191.9" / 72.0" / 58.0" compared to the Model 3 at 184.8" / 76.2" / 56.8". Note that the Altima's width is without mirrors.
Back to the track width: The 2017 Altima SR has the exact same size tires as the Model S - 235/45/18 and about 9.2" wide. So at the very least, the the outside of the wheels of both the Altima and Model S is within 0.2" with the Model 3 being slightly wider. The Prius Prime has a 195/65/15 tire which is 7.7" wide. So outside wheel width is about:
Model 3 track + tire: 71.4"
Prius Prime track + tire: 68.3"
Altima SR track + tire: 71.2"
We've now established that the wheels should basically sit in the same spot for the Altima and the Model 3 and the Prius Prime about 3" narrower. Now the question is: Do you really believe that the fenders of the Model 3's fenders stick out 2" farther on each side than the Altima? IMO that would really tuck the tires deep inside the fenders and would not be aesthetically pleasing. Even the Prius Prime's width of 69.3" is only about 1" wider than the track-width + tire width.
Given the above, it's my opinion that the mirrors stick out 2" past the fenders on the Model 3.