What is the meaning of "v'Im Yiskefo ha'Echad"?
Rav Sadya Gaon: If someone (Rashi - a bandit) will grab one of them.
Ibn Ezra 1 : If someone will grab and overpower 2 one of them. It should say Yiskefehu or Yiskenu; the Hei or Nun is omitted.
Ri Kara: If one tries to kill him, the father and son will oppose him.
Seforno: If one disputes them with a fallacious claim? (refer to 4:12:2:2).
Ibn Ezra: Some say that The Vov at the end of Yiskefo is extra; it should say Yiskof. This is difficult. We find an extra Vov at the end only for nouns, e.g. "l'Mayno Mayim" (Tehilim 114:8), "Bno Be'or" (Bamidbar 24:3).
Metzudas Tziyon: Yiskefo refers to holding strongly, like "v'Lo Yuchal Ladin Im she'Takif Mimenu" (10:6).
What will result from two standing against him?
Rashbam: One cannot defeat two people.
Seforno: They will be able to show [one who disputes them] his error, and draw him to the truth.
What do we learn from "veha'Chut ha'Meshulash Lo vi'Mherah Yinasek"?
Rashi: If a Chacham's son and grandson are also Chachamim, Torah will not cease from his descendants - "Lo Yamushu mi'Picha umi'Pi Zar'acha umi'Pi Zera Zar'acha" (Yeshayah 59:21). 1
Rashi, from Yerushalmi Pe'ah 1:9: If one [learns] Mikra, Mishnah and [acts with] Derech Eretz, he will not be quick to sin. This is only if he does not retract - "veha'Chut ha'Meshulash Lo vi'Mherah Yinasek." It does not say that it will never be undone.
Menachos 43b: Anyone who has Tefilin on his head and arm, Tzitzis on the garment and Mezuzah on the doorway, he is firm that he will not sin - "veha'Chut ha'Meshulash Lo vi'Mherah Yinasek."
R. Avigdor citing Koheles Rabah: If the two have a Rebbi, he makes them retract from their mistakes.
Yerushalmi Pe'ah 1:1 If one guarded himself from sin once, twice and a third time, from now and onwards Hashem guards him. This is only if he does not retract [from his virtue] - "veha'Chut ha'Meshulash Lo vi'Mherah Yinasek." It does not say that it will never unravel. If one stretches it, it will snap.
Rashi: [Two are better than one, and] all the more so, if there are three, and a bandit seizes one of them, they will stand against him.
Ibn Ezra: If the "Sheni" is "Ezer [k'Negedo]" (Bereishis 2:20, i.e. a wife), ha'Meshulash is the son.
Rashbam: If three friends join together, their bond will last.
Ri Kara: All the more so, if there are three (a father and two sons, one will not be quick to oppose them.
Seforno: If three debate to establish the truth of a matter, it is rare that they will argue 2 . If there are two opposite opinions, the third will mediate between them.
Metzudas David: This is a Mashal. If three join together, there is nothing better than this.
Kesuvos 62b learns so from our verse. Koheles Rabah - if the three are Soferim, or Bnei Torah, the family will establish more Soferim or Bnei Torah. If they are Ashirim, it will establish more Ashirim. It does not say that the rope will never unravel. If they become haughty, the wealth will snap.
And not reach a conclusion. (PF)