5D3 can do 60 fps at 720p
This iPhone offers something unique in the sense that it can do slow motion in such a small package and also has decent optics and image stabilization.
Your base cost is $400 if you get the 64gb.
5D3 can shoot amazing quality footage and be outfitted with cinema lenses which cost 25,000 for just a lens for example.
The base cost for the camera with a basic lens, expect to shell out $4,000
The sensor is obviously more powerful, more sensitive, and geared toward professional use.
Now quality can be subjective.
You can give a 5D to a hobbyist and an iPhone 5s to a filmmaker and the iPhone could outperform the 5D with proper shooting technique, lighting and color grading.
Now if you are wondering if the footage can intercut with 5D footage, it really depends on the lens on your dslr, but they have very different looks in color and contrast. You can't shoot flat (cine style) to get greater latitude in the image.
If you are trying to use an iPhone in place of a 5D then good luck. Hope this clears some things up.