To take a screenshot or screen capture on the Samsung Galaxy S3:
Hold down the HOME button and the POWER button simultaneously.
Enable palm swipe via the MENU button, select SETTINGS > MOTION. Check the ‘Palm swipe to capture’ box under Hand Motion settings. Move the palm of your hand on the screen from either left to right or right to left.
All screenshot images taken using the methods described above are saved by default to the Pictures> Screenshots folder on your phones internal SD card. You can also see the captured images in the picture Gallery. You can preview the latest screenshot image in the Notification Panel by swiping your finger down from the status bar.