Your DISCOVERY 3 features reverse charging technology, which allows it to function as a portable battery.
To use this feature, simply use the USB-C cable included with your device and connect it to your DISCOVERY 3; at the other end of the USB-C cable, simply connect the device you want to charge.
When you connect your DISCOVERY 3 to another device that is compatible with this technology, an additional option will be displayed in the USB preferences to select whether you want the DISCOVERY 3 to be the device being charged or whether you want the other device to function as a portable battery.