AMD and Intel are not the only ones manufacturing usb3.x controllers. There's also ASMedia, Etron, Fresco Logic, Renesas, Texas Instruments, VIA.
Because of that, there's no such thing as a universal usb3 driver.
I don't know for certain, but I can assure you that The Rain doesn't integrate just one driver.
Also, apart from the USB3 drivers, you'll need the SATA AHCI/RAID drivers for the various controllers.
All of the above mentioned drivers need to be integrated both into the 2nd index of the boot.wim and into all indexes of the install.wim.
Personal opinion: Don't waste time combining x86 and x64 into one iso. Everything since Core 2 Duo / Pentium Dual Core Exxxx and the 65nm Athlon64 X2's can run 7 x64 without a hitch, even at 2GB of RAM. Go with x64 only.