IF you are a Xiaomi Mi 3 owner and want to take screenshots or screencaps without downloading a app then you could do the following instructions:
Locate the power button (it is on the right side of the phone) and volume down button (also in the right side above the power button.
Once you have located it you need to press it both simultaneously for about 1-2 seconds.
You will notice a flicker on the screen, a sign that a screenshot was captured.
You can locate the screenshots on the album folder.