To take screenshots with the Samsung Galaxy S3 Android phone, is only a matter of a gesture with you hand.
First we have to enable it at:
Settings -> Motion -> Palm swipe to capture
And also be sure that in the former step, Motion is also enabled:
Now that it is enable, just swipe the screen with your palm, from right to left or left to right. Just if you were cleaning the Galaxy S3 screen, and the screenshot will be taken, and put in your gallery.
If you have Dropbox enabled, it will be uploaded there, for you to share it easily