I am assuming that you received a conviction because in your question you stated you had probation. If it is a conviction you can have it sealed but I doubt you can have it expunged. Keep in mind if sealed private entities may not be able to see the criminal charge but legal agencies can see the charge.
Many people confuse probation with court supervision.
Retail Theft (720 ILCS 5/16 A-3) Can be Expunged/Sealed 5 years after the Completion of court supervision.
Also you must have not been placed on supervision or convicted of another crime since then AND not have any charges pending against you right now.
Actually, the three year/five year waiting periods are for cases which do not end in judgments of conviction. That is when supervision can be sealed and expunged, respectively. Since you had probation and not supervision, the conviction can be sealed after 4 years.
If you are not sure what disposition you received, please check out this guide i wrote on figuring that out:
I hope this helps.