When someone commits the crime of 3rd degree assault the 3rd degree assault is usually in most states classed and charged as a Class A Misdemeanor.
A class A Misdemeanor means that the person who is charged with 3rd degree assault would likely get 6 months to a year in jail.
They may also get 3 years of probation or even possibly just a fine and serve no jail time for the 3rd degree assault.
Of course if you've been convicted of other things or have been in trouble with the law and are already on probation and commit the 3rd degree assault then you might serve a few years in prison because you'd be breaking your probation.