Have a Kit Kat Galaxy S3 users!
While Samsung is still busy in pushing Android 4.3 Jelly Bean update to the Galaxy S3, Google has already released Android 4.4 KitKat update, and it’s going to take a shitload of time for Samsung to release an official update for the same for Galaxy S3.
Code for CyanogenMod 11, that’s based on Android 4.4 Kit Kat sources has been out for a while now. Developers have been building upon these code cooking their own AOSP based Android 4.4 ROMs or just releasing an unofficial CM11 build for various devices.
The Galaxy S3 too, has just received an unofficial CM11 build that you can flash via recovery just like any other CM build with an accompanying  gapps package. The ROM is working fine and most of the bugs have already been ironed out. If you’d like install Android 4.4 on your Galaxy S3, go ahead with the step-by-step guide below.

SAMSUNG GALAXY S3 GT-I9300  ANDROID 4.4 KITKAT UPDATE INSTALLATION GUIDE

DOWNLOADS

Download the files given below and transfer them to a separate folder on your phone and remember the location.

• CyanogenMod 11 KitKat: DOWNLOAD LINK
  File name: cm-11-20131112-UNOFFICIAL-i9300-PREVIEW.zip
  File size: 185.05 MB

• Google Apps: DOWNLOAD LINK
  File name: gapps-kk-20131101.zip
  File size: 125.4 MB

Reminder: Before you proceed, make sure you’ve transferred the files you downloaded above to a separate folder on your phone.

STEP-BY-STEP GUIDE

Make sure you have backed up your phone adequately (including important data stored on internal memory). Also create a nandroid backup using your recovery for additional safety.
You’ll need either CWM or TWRP recovery for installing the CyanogenMod Android 4.4 KitKat ROM on your Galaxy S3 GT-I9300. And since CWM and TWRP recoveries function differently we’ve put together separate guides for both of them. So if you’re a CWM user, follow the CWM users guide and if you’re a TWRP user follow the guide for TWRP users.
NOTE: Android 4.4 requires latest version of CWM and TWRP recoveries, otherwise the Android 4.4 ROM won’t install fine.

GUIDE FOR CLOCKWORKMOD RECOVERY (CWM) USERS

1. Boot into recovery mode. For this:
   • Power off your device and wait for 5-10 seconds until the device is fully switched off.
   • Press and hold  Volume Up + Home + Power buttons together and release them as soon as you see the Galaxy S3 logo.
     └ In Recovery mode, use Volume buttons to navigate Up and Down between options and use Power button to select an option.
2. Create a Nandroid Backup from recovery. It’s optional but very important to do, so that in case something goes wrong you can restore to current status easily. For making a Nandroid Backup, go to Backup And Restore » Backup.
3. Perform a Factory Reset (This will delete all apps and their settings and game progress). For this:
   • Select Wipe data/Factory reset, then select Yes on the next screen to confirm factory reset
   • Go back to the main menu of CWM and go to Mounts and Storage » Format System and select Yes on the next screen to confirm.
   • Go back to the main menu of CWM, again, and go to Advanced » Wipe Dalvik Cache and select Yes from the next screen to confirm.
4. Install the cm-11-20131112-UNOFFICIAL-i9300-PREVIEW.zip file first. For this:
   • Select Install zip » Choose zip from sdcard (or external sdcard, you know where your files are) » browse to the location where you saved the file and select the cm-11-20131112-UNOFFICIAL-i9300-PREVIEW.zip » select Yes on next screen to confirm installation of the file
     
5. Now similarly flash the gapps-kk-20131101.zip, as you did in step 4.
   └ Make sure to first flash the ROM file and then the Gapps file.
6. After you are done with flashing of the files, reboot your device. For this, go back to the main menu of recovery and select reboot system now.

That’s all. Your phone will now reboot and it will take some time as it’ll be phone’s first boot after installing Android 4.4, be extremely excited for this!

GUIDE FOR TWRP RECOVERY USERS

1. Boot into recovery mode. For this:
   • Power off your device and wait for 5-10 seconds until the device is fully switched off.
   • Press and hold  Volume Up + Home + Power buttons together and release them as soon as you see the Galaxy S3 logo.
     └ In Recovery mode, use Volume buttons to navigate Up and Down between options and use Power button to select an option.
2. Create a Nandroid Backup from recovery. It’s optional but very important to do, so that in case something goes wrong you can restore to current status easily. For making a Nandroid Backup, go to Backup » and select all checkboxes and swipe on the Swipe to confirm option at the bottom of the screen to confirm backup.
   
3. Perform a Factory Reset (this will delete all apps and their settings and game progress). For this:
   • Tap on Wipe » then at the bottom of the screen do a Swipe on the ‘Swipe to factory reset‘ option
4. First Install the ROM file cm-11-20131112-UNOFFICIAL-i9300-PREVIEW.zip:
   • Tap on Install » browse to the location where you saved the files and tap on the cm-11-20131112-UNOFFICIAL-i9300-PREVIEW.zip file. Now at the bottom of the screen, do a swipe on the Swipe to confirm flash option to begin flashing.                                                                                                                            
5. Now similarly flash the gapps-kk-20131101.zip file.
   └ Make sure to first flash the ROM file and then the Gapps file.
6. After you are done with flashing of the files, reboot your device. For this:
   • Go back to the main menu of recovery and tap on Reboot » then, tap on System to reboot your phone.

That’s all. You have successfully flashed CyanogenMod 11 on your Samsung Galaxy S3 GT-I9300. Enjoy!