I have a software in development which has has the ability to turn off your display among changing Windows power plans if you are plugged in or not, and add keyboard shortcuts to your Surface Pro (or desktop or laptop), and control the brightness of external monitor(s) connected to your laptop or Surface or even desktop (monitor must support DDC/CI protocol). Now, as this is a Surface Pro 3, which support connected standby, the feature to change Windows power plans based on if you are plugged it or not, is useless, as you only have Balance power plan. And I have no idea if the turn off screen will actually put the device to sleep, but if you are willing to give it a spin, let me know, and I'll give you a link to get it.