Is it possible to automatically put individual monitors to sleep when not in use?

***Native OS functionality (I'm not familiar with 3rd party apps of this kind)*

I suggest using the built-in win + P method to change the display type. Changing it to PC screen only will allow 2 of your 3 monitors to sleep.

Note: This will only allow you to switch from 3-monitors to one - but this is as close as it gets natively to the functionality you're talking about.

Not sure if there is anything built in to Windows 8 to handle this, but I know that UltraMon will do what you want.

From here: With Ultramon the solution is simple, you can rightclick on the Ultramon icon in your taskbar to see a predefined list of actions. One of those actions is "Disable Secondary". You can either just select it from there or map a key combo to it (which is what I did) and now you have the ability to just turn off your secondary monitor with a click, or combo key press. This effectively allows you to switch from dual display mode to single and back again immediately. My second monitor just blacks out into power saving mode when I use my key combo and restores when I activate dual display with the same key combo when I am ready to use it again.