Jusb3 is the SuperCharger port of your motherbd. For it to charge, you need either need to run the supercharger app or put the computer into standby or hibernation. Please note that no data can be transferred while the port is in supercharger mode. This is covered onpage 1-28 of your users manual if you need more info.
As mentioned. to charge while off you will need to use a powered usb hub that you, obviously, need to leave the power on for.