There are four ways to take screenshots on a Mac, and all of them can be done with easy keyboard shortcuts.
If you have a MacBook with a touch bar, you can use Shift + Command + 6 to take a screenshot of the touch bar.
If you press Shift + Command + 5 on your Mac’s keyboard, you’ll open the screenshot menu, allowing you to take screenshots and videos, as well as customize your settings.
By pressing Shift + Command + 4 you can take a quick screenshot anywhere on your screen.
source: BI
Just press shift + Command + 3 takes a screenshot of the entire screen.