It would only be possible to go to prison (over 1 year in custody) on a 3rd DUI on specific circumstances. Generally, a 3rd DUI is only a misdemeanor with a minimum of 120 days of jail and a maximum of 1 year in jail.
One only goes to prison for felonies, and a DUI is a felony only under certain circumstances. Specifically a DUI can be a felony if one has 3 prior convictions in 10 years prior to the offense date of the current 4th time DUI.
Any DUI can also be (though isn’t required to be) a felony if in the commission of the DUI offense, the suspect is responsible for causing an injury.
Lastly, one can get a felony DUI for any DUI committed within 10 years of just one prior DUI if that prior DUI was a felony. In other words, if one gets a felony DUI with injury, any subsequent DUI (whether a 2nd or 3rd, whether injury or not) could be charged as a felony.
Any of those felony DUIs could result (though don’t have to result) in custody time in prison, which means greater than 1 year. It is also possible to get formal felony probation and jail time, which means less than 1 year in custody in county jail.