As I wasn't able to solve the problem related to JOdin3, I rooted the device using heimdall and CF-Autoroot. It worked flawlessly. I installed Clockwork Recovery and I'm able to backup the system. Now, I can continue upgrading the system to CM 12.1. Anyway, heimdall-frontend seems to be more advanced than Odin.

Here's a tutorial for it: http://www.all-things-android.com/content/how-root-samsung-mobile-device

I installed CM 12.1 successfully and Android 5.1.1 is now running on my I9300. :)