This came with my motherboard (Asus P8z68-v). The manual says "these connectors are for the additional USB 3.0 ports... If the USB 3.0 front panel cable is available from your system chassis, with this USB 3.0 connector, you can have a front panel USB 3.0 solution."
The back USB panel is for USB 3.0. As was clearly stated in the manual they are for USB3.0 and if your case has front panel USB3.0 you can plug that into the 3.0 header and have front panel USB3.0 instead of using the back bracket.
The second cable is to adapt the USB 3.0 ports on the front panel to a usb2.0 header so you can still use them if your motherboard doesn't have USB3.0 headers.
The 3rd "dohicky" is so you can connect front panel USB that has.individual pinouts, if your case had come with them like that.
Plug the front panel USB3.0 into the header on the motherboard and throw the 3 adapters in a spare parts box you don't need any of them