You most certainly can plug whatever you want into a USB 3.0 port, USB 3.0 is backward compatible. However, the ports will not function at all until you install the proper drivers. It's always best to keep your mouse/keyboard in USB 2.0/1.1 ports for that reason.
USB 3 drivers are included in Windows 8.x. Any USB3 port should work automatically in Win 8 the way USB2 ports do in earlier versions of Windows.
The purpose of USB 3 is to speed up storage devices, so it's true that most other devices won't care about the difference between USB 2 and 3