Unable to root S3

So I was trying to root my gf's she today and tried to flash twrp using Odin and kept failing. I then went to recovery to just wipe the cache and noticed something I've never seen before. On the bottom of the recovery screen out mention something about manual mode and multi csc from at&t. Is this the reason why I'm not able to root? And of so what is the steps needed to take in order to be able to root it and possibly install a custom ROM?

Thank ya!



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