Add rotation toggle to powerbutton menu?
In the more recent updates to the Galaxy S III (at least Android 4.1.2) you can modify the notification panel at the top of the notifications screens that you get when you drag from the status bar. Open settings, 'Display' (under 'Device' heading) then in the 'General' heading choose 'Notification panel'.

Here you should be able to choose which Quick setting buttons you want to appear and one of them is the toggle for 'Screen rotation'.