Samsung Galaxy S Duos 3 is not receiving official update to Android Lollipop. You will need to root your device if you want to install Android Lollipop using any custom ROM.
If you are willing to root your phone, here are the steps on how to achieve it.
1. Make sure your phone has at least 70 to 80% battery.
2. Take a complete backup of all your important data.
3. Enable USB Debugging on your device. To do that, go into settings and then into Developer Options. You should see the tab for USB Debugging. If you cannot see Developer Options, go into About Phone and tap on the Build Number 7 times.
Steps to root
1. Download ODIN on your PC. You may get it from the link http://www.androidrootguide.com/list-android-custom-roms-official-roms-root-software-download-links
2. Download Galaxy S Duos 3 Root Package You can download it from above link.
3. Download Kingoroot.
4. Extract the ODIN zip package downloaded above and install it on your PC. Extract Galaxy S Duos Root Package and save the Sduos3ROOT.tar file to your PC.
5. Switch off your device and boot into download Mode. To do this, press and hold Power, Volume Down and Home buttons simultaneously.
6. Once the device boots into download mode, connect it to your PC using USB cable.
7. ODIN should now identify your device and show Added.
8. Click on PDA button and choose Sduos3ROOT.tar. Ensure F.Reset Time and Auto Reboot options are enabled on your ODIN.
9. Click on START to flash the device. Once flashing is done, you will get the PASS message on your ODIN.
10. Close ODIN and install the Kingoroot you downloaded above. Open the program. Click on Root to begon rooting your device. Once done with rooting, your device should reboot automatically.
That's it. You have successfully rooted your phone.
You may need to install Custom Recovery to be able to install a custom ROM on your phone. To do this, you need to follow these steps -
1. Download Galaxy S Duos 3 CWM recovery zip package to your PC.
2. Extract and save recovery.tar.md5 to your PC.
3. Follow the steps 5, 6 and 7 as in the above tutorial.
4. Click on PDA button and choose recovery.tar.md5. Ensure F.Reset Time and Auto Reboot options are enabled on your ODIN.
7. Follow step 9 above. Your device will reboot automatically.
You can now install any custom ROM based on Android Lollipop.
Please note that Rooting and Flashing custom ROM on your device is a risky process. Even a single wrong step can brick your phone. Also kindly note that rooting voids your warranty. If you do not have enough knowledge of the procedures involved, kindly do not proceed.
Live....and Let Live!