Just ran into this problem myself; in my case the standard print screen key worked on windowed JME3 apps, but not fullscreen. I searched & found this post, but the link @Chaotic provided seems to be dead. I was able to find the screenshot info in the JME3 wiki which states:
The com.jme3.app.state.ScreenshotAppState enables your users to take screenshots of the running game.
You activate this feature as follows in your simpleInitApp() method:
ScreenshotAppState screenShotState = new ScreenshotAppState();
this.stateManager.attach(screenShotState);
The default screenshot key is KeyInput.KEY_SYSRQ
, also known as “System Request / Print Screen key. On Mac keyboards, this key does not exist, so on Mac OS you take screenshots using Command+Shift+3 (fullscreen) or Command+Shift+4 (windowed: press space to select a window and then click).
The screenshot is saved to the user directory.
Version Last updated 2016-07-22 07:15:15 UTC