A third-time DUI conviction in Florida is a third-degree felony if the third conviction is within ten years of any previous conviction. A third degree felony carries up to 5 years in prison. However, this is not the same thing as a mandatory prison sentence. Most of the time, a person convicted of felony third-offense DUI will get some local jail time and a lot of probation. Many times, the state will charge the case only as a misdemeanor even if the most recent conviction was within ten years. This is because most felony prosecutors haven't tried a DUI case in years. DUI cases are very complicated, and most felony prosecutors would rather keep them in county court as a misdemeanor, rather than having to re-learn all of the DUI law and procedure that they had to learn when they were just starting out.
In short, a third offense DUI can carry a prison sentence. However, the vast majority of felony DUI defendants do NOT go to prison.
It says from your question that you were convicted of a 3rd offense is that accurate if so you should have been sentenced already. On a third offense within 10 years of the second the State can charge you with a felony. It is a 3rd degree felony where punishment is a maximum of 5 years in Prison. Depending on the county you could receive prison time. Although unlikley it is possible.
The other attorneys are correct. If you have been convicted of a 3rd DUI within 10 years, it is a 3rd degree felony, and you face a maximum prison sentence of five years. However, there is also a mandatory-minimum sentence of 30 days jail, along with all of the other DUI penalties (probation, license suspension, community service, fines, etc.). In my experience, while defendants rarely face the maximum sentence, the same could also be said about the minimum. Your sentence will depend on the prosecutor, the judge, your sentencing scoresheet, and the individual facts of your case. Good luck.