Android is linux based, and the paradigm there is "free RAM is wasted RAM". So it's normal for any linux-based system (or any unix variant) to run with high RAM usage. Frequently or recently-used apps will be cached in RAM, rather than cleared out. This means that if you return to them they are already loaded, giving faster response and using less power than if they had to be reloaded from storage. Should the RAM be needed for a different app, the OS will clear one of the inactive apps from memory, but there is no advantage in doing this pre-emptively. So to me what you describe doesn't actually sound like a problem.
If you are seeing an actual performance hit after the JB update then I'd suggest asking in the S3 forum here, where other users with experience of that phone and software can advise. But otherwise I'd not worry about it at all
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