Here's the Illinois Code on burglary:
ARTICLE 19. BURGLARY
(720 ILCS 5/19‑1) (from Ch. 38, par. 19‑1)
Sec. 19‑1. Burglary.
(a) A person commits burglary when without authority he knowingly enters or without authority remains within a building, housetrailer, watercraft, aircraft, motor vehicle as defined in the Illinois Vehicle Code, railroad car, or any part thereof, with intent to commit therein a felony or theft. This offense shall not include the offenses set out in Section 4‑102 of the Illinois Vehicle Code.
Burglary is a Class 2 felony. A burglary committed in a school or place of worship is a Class 1 felony.
(Source: P.A. 91‑360, eff. 7‑29‑99; 91‑928, eff. 6‑1‑01.)
I'm not sure how you were charged with a Class 3 felony, when it is listed as a Class 2 felony. Nevertheless, under Illinois’s laws, a class 3 felony is punishable by two to five years’ imprisonment, while an extended term class 3 felony is punishable by five to ten years in prison. (730 Ill. Comp. Stat. § 5/5-4.5-40.) An extended term can be given by judges for a variety of reasons, such as if a person has a prior criminal record, or if the victim was over 60, for example. In Illinois, conviction for a class 2 felony can result in a prison term of three to seven years, or seven to 14 years for an extended term. (730 Ill. Comp. Stat. § 5/5-4.5-35.).
There is no minimum mandatory sentence, however, meaning a judge could also give a person probation, like I was saying.
If you do not have a lawyer yet, you will at least want to consider having a consultation with a criminal defense lawyer before your first court appearance. Many offer free or low cost consultations, and there is no obligation to hire them. However, a lawyer can go over all of the facts with you and give you a better idea of what they feel the outcome would be, possible issues and defenses, etc.
Your first court appearance will be your arraignment, where you are expected to enter a plea to the charge. There's never usually any benefit to pleading guilty since the state has the burden of proof anyway, so you may wish to plead not guilty (you can always change your plea later if advised to do so by your lawyer and you feel it is in your best interest). The court will then ask you if you have an attorney or you can tell the court you need a lawyer if you cannot afford one.
Your lawyer can request all of the evidence that the state has (called "discovery") which will help determine how strong a case they have against you and your best approach. If there doesn't appear to be a strong defense, they may discuss with you the idea of negotiating with the prosecutor to reduce the charge and/or minimize the penalties against you.