Unzip the file you get. Tar.md5: GT-I9300_XEO_I9300XXELKC_I9300OXXELK4_I9300XXELKC.tar.md5.
Now open Odin v1.85 ( download here ), click PDA, then add the file. tar.md5.
CAUTION: Make sure the box re-partition is not checked.
Start by putting out your galaxy s3 and put the Download mode. (Simultaneously press the Power button, Home, and Volume Down. When a message appears, confirm by pressing the Volume Up)
Now connected to your PC (via USB cable) and press start.