An Odin flash will wipe everything off the internal storage.
If you're trying to factory restore with CWM and it's already flashed to your device just hold vol down, power, and home while booting, then wipe you data, cache, and dalvik cache (in advanced).
If you don't have CWM installed then just straight up buy ROM Manager and use that to flash/boot into recovery. You'll thank me later.
Galaxy SIII - Thanks Appreciated!
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