This Root process wont erase device internal storage or information from SD memory card.
But it’s still strongly recommended to backup your data to device SD memory card or PC before starting this tutorial. In worst scenario, if device gets stuck on boot, it’s possible to lose everything that’s important for you.
Before we begin:
– The device should have at least 80% charge left on the battery.
– If you have Samsung Kies program in PC, Kies should be completely turned off to not disturb all the process.
– USB drivers must be installed on PC (if Kies are in PC then drivers are already installed).
– Download ODIN tool to PC.
– Download CF-Auto-Root-m0-m0xx-gti9300.zip to PC and unzip it.
– USB Debugging must be enabled on the device to transfer files between the phone and computer. To enable it go to Settings/About phone and keep taping on Build number until you see Developer mode has been turned on. After that go to Settings/Developer Options and mark USB Debugging.
Note: This tutorial is only for Galaxy S3 i9300 model. We do not recommend to try this Root on any other device.
Tutorial:
1. Galaxy S3 must be turned off.
2. Boot Galaxy in Download mode: Press and hold Volume Down, Home and Power buttons, all at the same time, release the buttons only when the ‘Warning!!’ message appears. Press Volume Up button to confirm that you wish to enter Download Mode. 
3. Launch Odin and connect device via USB cable to your PC. Wait for Odin to recognize it. Blue box should then appear in the program. 
4. In Odin, make sure that the Auto Reboot and F. Reset Time options are checked while Re-Partition must stay unchecked. 
5. Click on the AP button and select CF-Auto-Root-m0-m0xx-gti9300.tar.md5 file which was unzipped.
6. Click Start button in Odin to begin the update process.
7. After the process is complete, Galaxy S3 will restart and a green PASS message will appear in the left box at the top. 
8. The phone will restart. First startup can take up to 5 minutes, so be patient.
If you have any concerns at startup, perform a wipe data/factory reset in recovery mode (Volume Up + Power + Home button).
9. Turn off Odin and unplug USB cable from device.
Congratulations on successfully rooting Galaxy S3 i9300.
