This can be considered somewhat dangerous. Flashing bootloaders is scary stuff and if something goes wrong during a bootloader/chain flash, you're probably going to end up bricked.
If you get to a point where you don't want to go any further, or things don't work, you can always flash the stock rom using Odin in your bootloader mode.
Get that here: Download (unzip first)
This method is known to work on all versions of the Verizon SGS3 up to VRBMB1. It probably works on newer versions than that too.
I do, however, recommend that you downgrade to VRBLK3 before proceeding. That should hopefully prevent problems that can arise on newer ROM versions. Download VRBLK3
I want to give a quick intro into what we are going to do here. Hopefully it will help out with questions that people have while attempting this process.
As we all know, Verizon thought it to be a good idea to have Samsung lock the bootloader on this device. (see my message for Verizon at the top of this post)
What the locked bootloader prevents us from doing is flashing, via odin, a custom recovery or kernel. What we can do however, is revert to an older (semi-insecure) bootchain which we have acquired, flash our custom recovery, and then continue on from our freshly flashed custom recovery by flashing the fully unlocked bootloader and anything else we want.
(I know it looks like a lot of files, but they are relatively small. Just don't rename any of them)
ROOT AND RECOVERY
1. Copy Root.zip and VRALE6.bootloader.zip to your sdcard, for use later. (downloads 1, 6)
2. Install the Samsung drivers (download 2)
3. Boot into Odin mode on your device. (Power off the device, hold Vol- Home and Power until you get the warning message, then press Vol+)
4. Flash VRALEC.bootloader.tar.md5 (download 4) via odin (download 3) in the PDA slot. Your phone will reboot.
5. Boot back into Odin mode and flash a custom recovery (download 5) (also in the PDA slot) such as cwm touch or twrp:
6. Boot into your new recovery mode by turning off your device and holding Vol+, Home and Power
7. Flash root.zip and VRALE6.bootloader.zip from step 1 in recovery mode
WHAT TO DO NOW?
Back up your IMEI: http://forum.xda-dev...d.php?t=1852255
^That may save your ass later, trust me ^
Reset your flash counter (optional):
See here: http://forum.xda-dev...d.php?t=1494114
What is the flash counter you ask? Its a little number that gets incremented each time you flash (via odin ONLY, recovery WILL NOT increment this) a non official image. The idea is that Samsung would be able to tell if the device had been running custom software at one point in time. The app linked above will reset this to 0, but you must still be rooted in order to use it.
BACK TO STOCK
Reset flash counter first using Triangle Away: http://forum.xda-dev...d.php?t=1494114
Flash the stock bootchain VRBLK3 via PDA in Odin: Download - Mirror
(The bootchain flash is to ensure that it will flash the entire stock tar.md5, the unlocked one might not work to do this)
Flash the stock rom via PDA in Odin: Unzip first!
Consider everything below this line legacy, the above method is much easier to achieve root and unlocked bootloader
Edited by invisiblek, 12 May 2013 - 08:32 AM.