It wont because of a few reasons.
Computers are not designed to receive power over the USB port to charge the battery, only the main power connector has the ability to do that.
The voltage and amperage of a USB port is low. Most laptops battery's voltage ranges from 11volts to 19volts, while USB is 5 volts, you can only charge devices at 5 volts or less with a USB cable.
The milli amps on a USB charger is significantly less then what is needed to recharge a battery, USB chargers tend to range between 500-2000 milli amps, resulting in 2.5-10 watts of power which is extremely low for charging something like a laptop. While the Surface Pro 3 does use a smaller charger than bigger laptops, USB if it could charge will not work very well.