USB 3 in a USB 2 port will only work at USB 2 speeds, so a USB port is best to take advantage of USB 3 speeds.
USB 2 will still only work at USB 2 transfer speeds in a USB 3 port.
Most USB 3 ports and plugs have a blue separator component that can be seen to identify it as USB 3.
On Windows USB 3 is usually called "enhanced" USB in the "Device Manager" listings listings