Hi:
Did you try this tool? This normally takes care of the issue...puts the USB3 drivers into your W7 installation flash drive.
There is a readme file on the page containing the instructions for use. You also need to run it on a PC with W8.1 or newer according to the info on the page.
https://downloadcenter.intel.com/download/25476/Windows-7-USB-3-0-Creator-Utility
