The iPad Pro's (only 10.5 and 12.9) have 16 pins in the port for that capability and Apple even discloses that USB 3 speeds only are achieved on those specific models, the 9.7 is excluded because it and all other lightning devices still only have 8 pins. Also the iPad Pro's, 10.5 and 12.9 only, were specifically called out to have USB-C controllers that allowed them USB 3.0 function and those have not appeared yet in any iPhone. There are postings around that have confirmed that when connected with the Lightning to USB-C cable, the iPhone's still only report back USB 2.0 speeds.
My guess is that the USB 3 relies upon the specific controller and 16 pins in combination, which currently no devices have besides the 10.5 and 12.9 iPad Pro's.
I read that Apple's current iPhone 7 lightning port may have transfer speeds up to 480 megabits per second. Apple doesn't post their speeds. So anyone here claiming otherwise is only speculating. Just like me.
BTW, 480 mbps is about USB 2 speeds.
But Apple is more innovative and was ahead of the competition. Right?