iPhone 7 and iPhone 7 Plus are USB 2.
Besides, the only accessories Apple makes that support USB 3 are the USB-C to Lightning cable, and the 2015 revision USB 3 Camera Connection Kit and SD card reader.
This means that even if the 7 did support USB 3 you probably wouldn't have a cable that would support that speed anyway, unless you had a USB-C-equipped PC/Mac.
(And note also that the iPad Pro 12.9" does support USB 3 but I've had no luck getting it to sync at USB 3 speeds even with a USB-C cable.)