English Explanation of Mishnah Sanhedrin 4:3

Introductio Mishnah three begins to discuss the physical arrangement of the sanhedrin, the scribes who would record the decisions and the disciples of the Sages who observed the proceedings and learned.
The Sanhedrin was arranged like the half of a round threshing-floor so that they all might see one another.
Before them stood the two scribes of the judges, one to the right and one to the left, and they wrote down the words of them that favored acquittal and the words of them that favored conviction. Rabbi Judah says: “There were three: one wrote down the words of them that favored acquittal, and one wrote down the words of them that favored conviction, and the third wrote down the words of both them that favored acquittal and them that favored conviction.
The sanhedrin of twenty three that would try capital cases and the sanhedrin of seventy one would sit in a half circle. This was the seating arrangement that would best allow all of the judges to see each other. A full circle would mean that the one testifying before the court would have his back to some of the judges.
The second half of the mishnah describes the court stenographers. According to the first opinion, there were two scribes who recorded the court procedure, one which recorded the opinion of those that favored acquittal and one those that favored conviction. Rabbi Judah claims that there was a third scribe who recorded all of the opinions. In this way there would be two copies of all of the decisions made.