My boyfriend was arrested in dallas county on a 3rd offense theft charge and he is on probation I'm hooeing for the best but preparing for the worst... so I'm asking what do you all think could happen what should I expect

That's not enough information, but I would prepare for the worst, sorry.
We don't know if he stole candy bars or cars, hurt anyone, threatened anyone, how far apart the convictions are, or even how crowded the prison is or what judge he might get.

Ask him, assuming he has had a talk with his lawyer.

What was the crime he is on probation for ?

What did he actually do to get charged ?