GDR3 has not yet released for Lumia Devices.. it would come in the next few weeks.
But you can signup in the app studio and download GDR3 preview. Both official release and developer preview will be same. Size is about 35 mbs
Welcome to WPC it should come in a few weeks or longer if you do decide to put the preview of GDR3 remember that you need a developer account and that it voids the warranty on your device.