If a computer case has a USB port in the front, will it support USB 3.0 if my motherboard supports it? Or does the case vendor need to specifically mention USB 3 support? (BTW, I haven't seen such case yet.)
USB3 requires new connectors, with more connections in them. These are backwards compatible, Therefore a USB1/USB2 device will plug into a USB3 host, but this means that unless your case has the new connectors, you can't in a USB3 device. You could still use these connectors for USB1/USB2 devices, or use USB3 devices in a USB1/2 compatibility mode.
I was discussing this issue today because I purchased an external USB3 hard disk. My motherbord supports USB3 and my case has two USB3 connectors in front. The other two in the back are those of the motherboard. When I connect my USB3 external disk to front ones, I get a message saying:
The Device can perform faster if connected to USB3 port
Finally we realized that the motherboard haw no extra USB3 plugs for connection to the front of the case. So,They connected the two USB3 in front with the spare USB2 connectors existing in the motherboard.
Hope you understand that although the motherboad supports USB3 and the front of the case has two USB3 ports, those two of the motherboard are available in the back panel of the case only.
For a front port to support USB3 the case port itself must support USB3 and it must be connected to a USB3 port on the motherboard.
In the early days of USB3 motherboards typically only had two USB3 ports, both on the back. At that time the only way for a case to support front USB3 was to have a cable come out the back and plug into the motherboard.
Later an internal header was introduced to allowcases to provide front USB3 without this hack.