It took me a long time reaching the Arizona statutes and administrative code to track down what you were cited for. My guess is Section R12-4-313 - Lawful Methods of Take and Seasons for Aquatic Wildlife.
Section R12-4-313 - Lawful Methods of Take and Seasons for Aquatic Wildlife
A. Subject to the restrictions of this Section, a person may take aquatic wildlife during the day or night using artificial light as prescribed under A.R.S. § 17-301. When a fish die-off is imminent or when otherwise deemed appropriate, the Commission may designate a special season by Commission Order to allow fish to be taken by hand or by any hand-held, non-motorized implement that does not discharge a projectile.
B. A person who possesses a valid Arizona fishing license may take aquatic wildlife by angling or simultaneous fishing as defined under R12-4-301 with any bait, artificial fly, or lure subject to the following restrictions:
1. Except for sunfish of the genus Lepomis, the flesh of game fish may not be used as bait.
2. Live baitfish, as defined under R12-4-101, may only be used in designated areas prescribed by Commission Order and designated areas may subsequently be closed or restricted by Commission Order.
3. Waterdogs may not be used as live bait in that portion of Santa Cruz County lying east and south of State Highway 82 or that portion of Cochise County lying west of the San Pedro River and south of State Highway 82.
4. Shall not use more than two lines at any one time.
5. The Commission may further restrict the lawful methods of take on particular waters by designating one or more of the following special seasons by Commission Order:So from your post, you did indeed violate R12-4-313 by casting out that third line.R12-4-301
“Simultaneous fishing” means taking fish by using only two lines at one time and not more than two hooks or two artificial flies or lures per line.
17-341. Violation; classification
A. It is unlawful for a person to knowingly purchase, apply for, accept, obtain or use, by fraud or misrepresentation a license, permit, tag or stamp to take wildlife and a license or permit so obtained is void and of no effect from the date of issuance thereof.
B. Any person who violates this section is guilty of a class 2 misdemeanor.I would not be pleading guilty. Find an attorney. Call the state's bar association and find one that is familiar with fish and wildlife regulations.13-707. Misdemeanors; sentencing
A. A sentence of imprisonment for a misdemeanor shall be for a definite term to be served other than a place within custody of the state department of corrections. The court shall fix the term of imprisonment within the following maximum limitations:
1. For a class 1 misdemeanor, six months.
2. For a class 2 misdemeanor, four months.
3. For a class 3 misdemeanor, thirty days.
B. A person who is at least eighteen years of age or who has been tried as an adult and who stands convicted of any misdemeanor or petty offense, other than a traffic offense, and who has been convicted of one or more of the same misdemeanors or petty offenses within two years next preceding the date of the present offense shall be sentenced for the next higher class of offense than that for which the person currently is convicted. Time spent incarcerated within the two years next preceding the date of the offense for which a person is currently being sentenced shall not be included in the two years required to be free of convictions.
C. If a person is convicted of a misdemeanor offense and the offense requires enhanced punishment because it is a second or subsequent offense, the court shall determine the existence of the previous conviction. The court shall allow the allegation of a prior conviction to be made in the same manner as the allegation prescribed by section 28-1387, subsection A.
D. A person who has been convicted in any court outside the jurisdiction of this state of an offense that if committed in this state would be punishable as a misdemeanor or petty offense is subject to this section. A person who has been convicted as an adult of an offense punishable as a misdemeanor or petty offense under the provisions of any prior code in this state is subject to this section.
E. The court may direct that a person who is sentenced pursuant to subsection A of this section shall not be released on any basis until the sentence imposed by the court has been served.

