That all depends on your PO (probation officer). Each case is different, and each jurisdiction is different. Have you been honest with your PO? Are you in drug/diversion court? Remember, your PO has to be able to justify not only why he/she might have to arrest you, but also needs to be able to justify why they should keep allowing you multiple chances.
I was a PO, and three positive tests would not always mean a trip to jail for the probationer, there were usually a few other factors to be considered. Are you working? Are you otherwise complying with probation? It all adds up one way or another.