California Code, Code of Civil Procedure - CCP § 640

(a)The court shall appoint as referee or referees the person or persons, not exceeding three, agreed upon by the parties.

(b) If the parties do not agree on the selection of the referee or referees, each party shall submit to the court up to three nominees for appointment as referee and the court shall appoint one or more referees, not exceeding three, from among the nominees against whom there is no legal objection. If no nominations are received from any of the parties, the court shall appoint one or more referees, not exceeding three, against whom there is no legal objection, or the court may appoint a court commissioner of the county where the cause is pending as a referee.

(c) Participation in the referee selection procedure pursuant to this section does not constitute a waiver of grounds for objection to the appointment of a referee under Section 641 or 641.2.