As Moshe stood on Har Sinai and had been informed by Hashem that Bnei Yisrael had fashioned for themselves a molten calf, Hashem made the following offer: “Vayichar Api Bahem Viachlam Vaeseh Otcha Ligoy Gadol,” “Let me be angry at them and destroy them and I will make you (Moshe) into a great nation.” However, Moshe chose to defend his people and pleaded with Hashem to grant forgiveness due to Zechut Avot as the Pasuk states, “Zecher LiAvraham LiYitzchak UliYisrael Avadecha…” Rashi, quoting the Gemara in Brachot 32a which comments on the connection between God’s preposition to Moshe and Moshe’s response, states as follows:
“If they (Bnei Yisrael) will not be saved in their (the Avot’s) merit, what is it that you say to me “And I’ll make you a great nation;’ if a chair of three legs cannot stand before You when You are angry, Kal Vachomer, a chair of one leg.” Essentially, the metaphor that Moshe uses is to be understood as follows: If the merits of Avraham, Yitzchak and Yaakov are not sufficient to save the people from destruction, then how can Moshe’s merit be so great that he deserves to be made the leader of a new nation.
Rav David ben Shmuel Halevi, author or the Turei Zahav, raises the following question: Why didn’t Moshe attempt to defend the people by joining his merit with the merits of the Avot? It is true that if a chair cannot stand with three legs is most certainly will not stand with one leg, but it will most definitely stand with four legs (Avraham, Yitzchak, Yaakov and Moshe).
The Taz answers that Moshe understood that if the merits of Avraham, Yitzchak, and Yaakov as a group have always been successful in maintaining and sustaining the covenant that Hashem has formed with the Jewish people (Eretz Yirsrael, Yetziat Mitzrayim), then if the situation is such that this Zcchut Avot is not sufficient, it is evident that the addition of the merit of one or more Av will be ineffective.
The Taz compares this to a well-known Talmudic concept known as Trei Kimeah – two are like one hundred. If, for example, two men have testified in a Jewish court to a certain fact, then if two other witnesses contradict the first pair, then both testimonies are rejected by the court. This rule holds true even if the first pair of witnesses come as part of a group of one hundred. The testimony of a hundred witnesses has no greater validity in Halacha than the testimonies of two witnesses. Since the testimony of two witnesses is always valid in other situations, it follows that the addition of ninety-eight others will be ineffective to outweigh the second pair of witnesses. Similarly, in our Parsha Moshe understood that if Zchut Avot, which is usually effective would prove to be to ineffective to cleanse the Jews from the Chet Haegel, then Moshe’s Zechut would be of no avail as a fourth leg to support the chair.