Sanhedrin 9A

דאתרו ביה מלקות, ולא אתרו ביה קטלא.

the witnesses warned her that she would be liable to receive lashes, but they did not warn her that she would be liable to receive the death penalty. In that case, the court would try her for adultery, and if found guilty she would receive lashes and not the death penalty.

וקמיפלגי בפלוגתא דרבי ישמעאל ורבנן, דתנן: מכות בשלשה. משום רבי ישמעאל אמרו: בעשרים ושלשה.

And they disagree with regard to the issue that is the subject of the dispute between R' Yishmael and the Rabbis, as we learned in the Mishnah: One who is accused of violating a prohibition that would render him liable to receive lashes must be judged by 3 judges. In the name of R' Yishmael it was stated: Cases involving lashes must be adjudicated by 23 judges. Therefore, R' Meir holds that the case of the defamer may be adjudicated by 3 judges, because he holds that a court of 3 may administer lashes. The dissenting opinion, which holds that lashes may be administered only by 23 judges, also holds that this case must be adjudicated by a court of 23.

רבינא אמר: כגון שנמצא אחד מן העדים קרוב או פסול, וקמיפלגי בפלוגתא דרבי יוסי ורבי, אליבא דרבי עקיבא. דתנן: רבי עקיבא אומר, לא בא שלישי אלא להחמיר עליו, לעשות דינו כיוצא באלו.

Ravina says a different explanation: The case in the Mishnah is discussing a situation where one of the witnesses is found to be a close relative or a disqualified witness, but two valid witnesses still remain. And R' Meir and the Rabbis disagree with regard to the issue that is the subject of the dispute between R' Yosei and R' Yehuda HaNasi with regard to the opinion of R' Akiva. As we learned in a Mishnah (Makkot 5b) that R' Akiva says: When the verse states: “At the mouth of two witnesses, or at the mouth of 3 witnesses, should a matter be established” (Deuteronomy 17:6), the third witness is mentioned only to be stringent with him, to make his status like these other two witnesses. If a group of 3 witnesses is found to be conspiring witnesses, the third one might claim that his testimony was unnecessary and therefore did no harm. The Torah nevertheless imposes upon him the same strict punishment as his peers.