Are the charges from the State? I'm not sure how that would work because it's not the same in all areas and depends on a lot of different things. Usually, if someone has a relatively short sentence, a year or less, they would do that time in the county jail, especially a young first-timer, but you never know. But they are still the State's responsibility. If the county jail has the room, the State pays the county a set rate per day to house these inmates. We have a new, bigger jail here and we have quite a few State inmates, and also house a few from other counties whose jails are over crowded.
But don't bet on it! Although he may end up actually doing only around a year IF he earns all the credits he's eligible for, they're not going to assume he'll get them before he earns them and take that off the top. His sentence would be the three years minus time served minus any other credits he's already eligible for. In a State facility the credits they earn are strictly up to them. They are given sentence credits monthly - they get behavior credits only if they have had no write-ups or disciplinary problems. And these credits can be taken away by a Disciplinary Board for very serious infractions. And they receive sentence credits for working at a prison job. In our system all inmates were assigned to a job, and any that did not have a HS diploma or GED were assigned to the prison school as their "job" assignment. Their work supervisor could give or withhold work credits according to their job performance. They also receive a very small stipend for working which they can spend in the commissary for soap, toothpaste, junk food, etc.
And Janet is right. There are many more opportunities and actually a lot more "freedom" in a State prison than in a county jail, and a lot more opportunities to earn those sentence credits! In a county jail, the number one problem is usually idleness and boredom. A State prison is more like a small self-contained town with a school, library, post office, hospital, chapel and dining hall. The one I worked in had two big factories where the inmates worked, educational opportunities, a good library, a gym, art and music programs, several clubs and organizations, and recreational programs like basketball and softball. They even had several vocational schools where they could learn the skills to get a good job when they got out. We had programs in culinary arts, cabinet making, wood work, HVAC and horticulture. We even had a cosmetology school where they could actually get their State license before they got out! Plus in a State facility, the inmates are very carefully classified and separated by security levels so the axe murders are not in with the guys who wrote too many bad checks!