An all-powerful judge is not much different than a king. Therefore, Mosiah wisely sets up a system of checks and balances. He does this not with three branches of government but with a system of accountability by which judges can be tried for not performing according to the law which has been given. In this system, the lower judge may be judged of a higher judge and the higher judge may be judged by a group of lower judges.
An analogous system has been set up in latter-day church government. When a priesthood leader transgresses, he is judged by a higher authority. When a president of the high priesthood (member of the First Presidency) transgresses, he is judged by a group of lower authorities:
And inasmuch as a President of the High Priesthood shall transgress, he shall be had in remembrance before the common council of the church, who shall be assisted by the twelve counselors of the High Priesthood;
…Thus, none shall be exempted from the justice and the laws of God, that all things may be done in order and in solemnity before him, according to truth and righteousness. (DC 107:82-4)