1- Download I9003 DDKL3 2.3.6 Gingerbread package from link given below Or from sammobile.com to your computer. After this you need to extract the same in your computer.
2- Now you will have to enable the USB Debugging from the path “Settings > Applications > Development > USB debugging. Ensure that the USB Debugging option is ticked
3-turn Off the phone so that I9003 can be entered in the Download Mode. The Download mode mode can be entered by pressing Volume Down Key + OK button, and holding both simultaneously press the Power Button.
4-Once the device is entered in the Download Mode, connect the phone to the computer while keeping the Odin opened in your computer. After automatic driver installation process, ID:COM section in Odin will turn yellow indicating that you are now ready to proceed to the next step.
5-Now in Odin, click on “PDA” and select “I9003DDKP3_I9003ODDKPI_I9003DDKP4_HOME.tar.md5”. And click START
-Please note that once the firmware is updated successfully, phone will be rebooting. It will take around 5 minutes for the phone to get applied this firmware.
6-Disconnect the phone from computer and let it be ON.and then you need to turn off the phone and go to recovery mode by pressing CENTER,VOLUME UP and ON/OFF Button.
7-Once you enter into the recovery mode, choose the options Wipe data/factory reset and also you need to select Wipe Cache partition and then reboot your device by selecting reboot system now option.
Thats all you have updated your mobile to latest official firmware DDKP3.
Disclaimer: I should not be held responsible in any manner just in case if anything happens to your device which is very unlikely if you follow all the mentioned instructions properly.