It is cause of the faulty crappy stock samsung charger, basically the usb cable which is thin and long.If you have thick and short usb to micro usb cable which comes with nokia lumia phones or even previous samsung phones, use it.I too have an s3 and had the same problem.Found an old short thicker usb to micro usb cable in my house,used it and the problem is solved.Download an app called galaxy charging lite from the play store and monitor the current voltage provide by the charger, it should be more than 900mv to charge the phone within 2 and half hours.