When it comes to releasing kernel source code, Samsung wastes no time. The Android 4.3 rollout for the Galaxy S3 has only just begun, but interested developers can already download the relevant kernel source from Samsung's Open Source Release Center, and start releasing custom kernels and ROMs that work well with the official Android 4.3 update on the device.
Hit the source link to grab the Android 4.3 kernel source. The “update12” file is the one you're looking for, with version XXUGMJ9.