Trouble!!

allright folks i bought a samsung galaxy s3 one month ago and now its saying emergancy calls only this is my second phone as i lost my other to a car accident in december however i paid for this phone from a private seller it worked for a month or so and now emergency calls only has started coming up so i have searched the internet for the past 2 days and have seen that its possible to a: change the logic board at a cost of £130, or b: root the phone and change imei number to unblock it so what i have done is chose to try the unblock method instead of spending £130 on it incase it doesnt work and im left out of pocket anyway, but one thing stands in my way... when i go to root my device it doesnt root it either just fails or it says get PIT file for mapping im clueless as to what this is i downloaded everything i would have needed according to the tutorial i was using and then i done a little digging via google and search bars ect and as it turns out samsung have introduced the "knox" security feature so i presume this would be the reason why it wont root as i am using 4.3 jellybean but i cant seem to bypass it and others have done this can someone shed some light on this for me as i am allready out of pocket £200 for a phone thats as much use as a paper weight here is a list of what the tutorial said to use,

odin 3.09
specific gt-i9300 file pack

i followed the instructions word for word and it still wont work can anyone forward me onto a root method that is relatively simple and will work please

thanks davievtec