Your beloved Samsung Galaxy S3 is nearing the end of its life cycle. Sure, it can do most of what newer devices can, especially if you were lucky enough to receive the KitKat update, but your diminishing battery and scratched up screen have seen better days.
The Jelly Bean update brought a lot of cool features to the Samsung Galaxy S3, but one nasty bug remained. Something was causing "Android System" usage to be consistently high. Of course, the percentage of this process that is used is dependent on what you're doing at any given time, but it shouldn't be anywhere near this level with "standard" use (calls, texts, emails, light browsing, etc.).