Where can I get the stock Android Kernel  configuration for Galaxy S3 running ICS?
Recently helped someone figure this out for another device, and the same method will probably work here:

grep all of the configs for CONFIG_ANDROID to select only those configs that enable the various android kernel customizations.  Rule out any that have substantially fewer matching lines than others.

Then examine those that match (hopefully only a very few) to see which mention Samsung, or the codename of the device or something like that.