I put together a tutorial on how to unroot the galaxy s3 and get it back to stock (for warranty or to get stock updates)
Things you will need...
1. PC Odin
2. Your Firmware
1. Backup what you need to
2. Install ADB Drivers (Samsung Kies)
3. Put your phone in download mode - Hold Vol Dn + Power + Home for 10 seconds then press Vol Up to continue
4. Check if Custom Binary Download is NO, if Yes then proceed to step 6 if it says 1 count or something like that then go to step 5
5. Follow the thread below to reset the binary counter (or check my video tutorial to do so...) once reset get back to download mode
(Thanks to Chainfire)
6. Go to SamMobile.com and download the latest firmware for GT-I9300 (Galaxy S3)
7. The downloaded firmware would be a zip file right click and extract it to get a .tar.md5 file
8. Connect the phone to the pc via usb cable
9. Run Extract the zip file that you downloaded from the attachment below and run Odin3 1.85. exe
10. Make sure that only Auto Reboot and F.Reset Time are checked and click on PDA, select the .tar.md5 file that we just extracted
11. Now hit Start
12. Once the phone is up go to Settings -> About Device -> Status, check if the device status says NORMAL... if it does, then the device is unrooted if it still says MODIFIED continue...
13. Reboot into Android Recovery by holding Vol UP + Home + Power till you see Samsung Galaxy S3 displayed on screen...
14. Do a factory wipe and wipe cache then reboot
15. Check Download mode again, if 0 or NO you are done and the S3 can be sent in for warranty if it still shows 1 go to step 16.
16. If the counter stays at 1 it means u were on a non stock kernel... what u need to do now is root (since u have just unrooted you are now on a stock kernel) use triangle away and then flash the stock rom again.... the counter will then be 0 but status might show modified, in that case do a factory wipe and youd be back to a totally stock s3
Hope this guide helps someone
Ash aka Cursed4Eva