First of all, while Intel does release drivers through its Download Center website, you should be relying on your motherboard manufacturer's website as your primary source for drivers that have been, where necessary, tailored for or, at least, validated with your motherboard. Secondly, the standard driver set for Windows 8.1, like that for Windows 10, includes driver support for USB 3.0; there is no separate downloadable release from Intel (though there might be updates available via Windows Update).
I would suggest that you look for an updated BIOS from your board manufacturer. Failing that, you could try completely uninstalling the USB 3.0 drivers and then let them be (automatically) reinstalled from the Windows Update catalog (on the subsequent reboot).
Hope this helps,
P.S. Take a look at this article: All the Ways You Can Still Get Windows 10 for Free.
This message was posted on behalf of Intel Corporation
Thank you, N. Scott Pearson for your collaboration.
AceOfHeartz, as Scott mentioned, Windows 8.1* comes with native support for USB 3.0, therefore; there is no need to install or update USB 3.0 drivers unless you uninstall those drivers from device manager, in that case, they will be downloaded and installed again through Windows Updates.
I recommend following Scott's advice regarding the BIOS update.
Hope this information helps.