Here is the video guide:
Which covers everything you need to know!

Useful links:
–Faryaab’s post on XDA with a collection of STOCK ROM’s
-Triangle Away by Chainfire – Paid version on play store & free version on XDA (Both are the same thing)
–Sammobile – where you can find a list of stock firmwares

Here is a written guide:
How to get into:
Download mode: Down + Home + Power keys
CWM Recovery: Up + Home + Power keys

-Download samsung drivers for the S3, via Kies for example
-Download firmware, relevant to you
-Un-Zip firmware and ODIN files -> you will get 2 folders
-Backup anything important, like SMS, contacts, apps etc, and save them to your PC
-Get Triangle Away app
-Use Triangle Away app to get into warning/download mode
-Let Triangle Away reset counter & reboot phone
-After reboot, switch off phone
-Get into download mode, and check if it has successfully reset the counter
-From download mode, go straight into recovery (don’t worry, the phone isn’t downloading anything, and you won’t damage it)
-Wipe data/factory reset, wipe cache, wipe dalvik
-Go into download mode
-Launch Odin, and setup stock rom MD5 file in PDA (do not touch anything else)
-Plug phone in
-Hit start & patiently wait for it to finish
-After phone has booted and you have fully booted up, unplug wire
-Check phone version is correct to what you flashed – in dialler type: *#1234#
-Get you IMEI number (keep this number confidential) by typing in dialler: *#06# – write this number on a piece of paper
-Get into CSC chooser & installer by typing in dialler: *#272*IMEINumber#
->If the CSC is wrong, or not right to your relevant region – Go here and get the right one for you
–>Install relevant CSC if it is wrong
-Wipe INTERNAL SD card, by factory resetting in settings, on your phone.
-Let phone reset, and reboot
-(optional) plug into Kies and let Kies upgrade your phone to the latest firmware
-Turn off phone and get into download mode
-Make sure you have no flash counter registered
-Reboot phone using power key
-Send into Samsung, sell and do as you wish on your “never flashed”, still valid for warranty Samsung Galaxy S III 🙂

On another note, you might be interested to know that, when on a custom ROM, and flashing another custom ROM via CWM, after using triangle away to reset the counter, your counter will not be increased.
For more information, watch this video:

Hope you enjoyed this extensive guide!
–TotallydubbedHD