this is my first thread im writing, yesterday i solved the issue of my S3 i9300, which was showing me no internet connection and invalid imei (0042xxxxxx0000) from past month after i updated my phone to 4.3. so i am just writing this post so that u do not waist your month for finding out difficult mathods to get back ur signals and waste your time like i did.
i tried Ariza patch, didnt worked
i tried almost evry modem, didnt worked
i tried root explorer to restore my efs folder.. imei was back but signal didnt
so the thing worked for me was installing just two files through CWM and everything is perfect...
what you need before installing these two files is a Rooted ROM in your galaxy S3( 4.0.4 , 4.1.2, 4.2.2 or 4.3 this patch workes with all) international, and a coustom recovery CWM, TWR or any other, after downloading the two attachments put them in your sd crad, reboot your phone to recovery, flash these two zips and then reboot again.
70% chances are that you will have every thing ok. but still if you are having problem with your emie and so that to your signals it means ur efs folder is damaged, in such case do write down and ill upload a link to restore efs folder. hope this will work for you guys, sorry for my bad english.