I think you just get differed and probably sent back in 3 months time or something. I'm not 100% sure, if you fail it your recruiter may suggest going into your second job choice or Infantry (If it isn't your second job choice)
It will all depend on what your ADSO put in your report, in most cases if you fail PRAC you will chat to your Recruiter who more than likely send you for another attempt at PRAC (dependant on your PRAC report) or just be able to click you over for your second choice. Worst case scenario would be for your Recruiter to speak to ADSO. Just put 100% in at PRAC and dont give up
Should you fail first time, dependant on the report the Sgt in charge of that particular PRAC gives you, you may be invited back for a second attempt. However a third, is not something I imagine would happen.