You can do 2 things at once on the S3 right now, (as long as one of them is watching video). The pop out video feature is pretty sweet but it's only available with the stock video player. Open a video with the stock player and tap the button in the bottom right corner. When the pop out screen appears, you can move it around and even resize it.
But for at least a while longer the only way to get the multiwindow feature is by installing a custom rom.