902.8  Minimum sentence — habitual offender.
  An habitual offender is any person convicted of a class “C” or a class “D” felony, who has twice before been convicted of any felony in a court of this or any other state, or of the United States. An offense is a felony if, by the law under which the person is convicted, it is so classified at the time of the person’s conviction. A person sentenced as an habitual offender shall not be eligible for parole until the person has served the minimum sentence of confinement of three years.
[S13, §4871-a, 5091-a; C24, 27, 31, 35, 39, §13396, 13400; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, §747.1, 747.5; C79, 81, §902.8]

See §901.5(7)