Android 4.1.2 is the latest Android firmware for Google Android and this firmware was official releases for Nexus 10 , LG Nexus 4 and other nexus flagship phones. Today, Samsung also official releases Android 4.1.2 firmware for international Galaxy S3 GTi9300. The firmware info is as below:
Base Firmware: I9300XXELKC(4.1.2)
Build Date: 30-11-2012
I am not sure when the firmware will reach your region. You can check and update the firmware by Samsung Kies or OTA update on your phones.But if you want to test the firmware now on your Galaxy S3, then you can manually download the firmware and flash into your Galaxy s3 via Odin. Here is the tutorial on how to flash Android 4.1.2 I9300XXELKC into Galaxy s3 GTi9300.
Note: Manually flash firmware into Galaxy S3 via Odin will void the phone warranty. So, take it on your own risk.
Step 1: Download GT-I9300_XEO_I9300XXELKC_I9300OXXELK4_I9300XXELKC (.zip), then unzip the file.
Step 2: On your computer- desktop or laptop, Open Odin 3.04 (Download Odin).
Step 3: Turn off your phone and then turn your phone in download mode by press (Home+power+vol down) button.
Step 4: Connect phone to the computer and wait until you get a yellow sign in Odin.
Step 5: Add Android GT-I9300_XEO_I9300XXELKC_I9300OXXELK4_I9300XXELKC (.md5) to PDA.
Step 6: Start flash, sit back and wait a few minutes.
Step 7: The phone will automatically reboot once installation complete.