Gemara

Kidushin 80b (Mishnah): A man may not be secluded with two women, but a woman may be secluded with two men.

R. Shimon says, even one man may be secluded with two women. If his wife is with him; they may even sleep together in an inn, since his wife will guard him.

Sotah 7a (Mishnah): Beis Din sends two Chachamim to accompany a man taking his wife (a Sotah) to Yerushalayim, lest he have Bi'ah with her on the way.

Including the husband, there are three men in all. This supports Rav Yehudah:

(Rav Yehudah): Two men may be secluded with a woman only in town. On the road, three men are required. If there were only two, if one needs to relieve himself, the other will be secluded with her!

Rejection: No. We send two so they can testify (if he has Bi'ah with her).

The Mishnah requires Chachamim to accompany them. This supports another teaching of Rav Yehudah:

(Rav Yehudah): Two men may be secluded with a woman only if they are proper. If they are lewd, even 10 men may not be secluded with her. A case occurred in which 10 men took a woman out in a coffin to have Bi'ah with her!

Rejection: No. We send Chachamim so they can warn him (that if he has Bi'ah with her, the water will not test her).

Kidushin 80b -Question: Why do we forbid a man to be alone with two women?

Answer (Tana d'Vei Eliyahu): It is because women are easily persuaded.

81a: Rav and Rav Yehudah were on the road. A woman was in front of them. Rav advised that they go in front of her.

Rav Yehudah: You permit two proper men to be with a woman!

Rav: Who says that we are proper? Perhaps only people like R. Chanina bar Papi are permitted!

Rishonim

The Rif (Kidushin 32b) brings the Mishnah and the distinction between proper and lewd men.

Beis Yosef (EH 22 DH veha'Rif): I do not know why the Rif omitted the distinction between the city and the road.

Rambam (Hilchos Isurei Bi'ah 22:8): A woman may not be secluded even with many men unless one of their wives is there. Similarly, a man may not be secluded with even many women.

Magid Mishneh: Since Rav did not consider himself and Rav Yehudah to be proper, no one (nowadays) is proper. Therefore, the Rambam forbids a man with many women.

Ran (33a DH Lo): Stam men not known to be lewd are considered proper. It was Midas Chasidus for Shmuel (our text - Rav) to tell Rav Yehudah 'we are not considered proper.' The Rambam holds that this was letter of the law.

Prishah (EH 22:8): It is difficult to say that the Mishnah discusses only individuals in the generation (like R. Chanina bar Papi).

Rashba (587): If Rav and Rav Yehudah were not proper, how would Beis Din find proper men to send with a man taking a Sotah to drink (Sotah 7a)?!

Gra (11): The Gemara forbids with lewd men. This implies that Stam men are proper. Rav was stringent to avoid thoughts of sin.

Rambam (ibid.): We are not concerned for many men with many women.

Bach (EH 22 DH Kosav): This permits two men with two women. If even this were forbidden, why did the Mishnah forbid only one with two?

Chelkas Mechokek (7): Perhaps 'many' means at least three.

The Rosh (Kidushin 4:21) brings the Mishnah and the entire Gemara.

Poskim

Shulchan Aruch (EH 22:5): A woman may not be secluded with even many men unless one of their wives is there. Similarly, a man may not be secluded even with many women.

Beis Yosef (DH Kosav ha'Rambam): We forbid one man with two women. Rashi explains that Leah is not afraid lest Miryam tell if Leah is Mezanah, for she assumes that Miryam will also be Mezanah. A woman is permitted with two men, for men are ashamed (to be Mezaneh in front) of each other.

Rema: Some permit a woman to be secluded with two proper men in the city. Stam men are proper. Seclusion with even 10 lewd men is forbidden. However, in the field, or at night even in the city, we require three, even if they are proper.

Ba'alei ha'Nefesh (Prishah 1:23, in Beis Yosef DH v'Chosav ha'Ra'avad): Some permit a woman with two proper men only during the day, but require three to sleep together at night. On the road, we are concerned lest one need to move his bowels. Likewise here, perhaps one will sin while the other sleeps! R. Yehoshua slept with a redeemed captive at his feet (Shabbos 127b). This shows that we do not need other women to guard the woman.

Hasagos Ba'al ha'Ma'or (5): We cannot learn from an episode with a great Chasid in pressed circumstances! However, whatever will inhibit sin suffices.

Maharik (98, cited in Beis Yosef DH Kosav Maharik): Later Poskim require three proper men. Some require three Chachamim. It is rampant for one woman to go with any three men, and people consider it to be permitted. Therefore, we do not excommunicate, dissociate from, or embarrass for this, unless they were warned.

Darchei Moshe (1): Mahari Veil (55) forbade a widower to hire a widow and her son to help in the house, for the son will not always be there. Surely he will go to the market or to roam, and his mother will be alone with the widower. R. Yerucham (23:1 192) requires three in the field only when the woman cannot be alone and needs a man to guard her. If not, when one goes to urinate, the other man will go with him, and leave her alone!

Beis Shmuel (9): We are not concerned lest the other man not go with him, for we discuss proper men. Nevertheless we forbid at night, lest one go to sleep. Even though he could wake the other, (if he does not) he is alone her. However, the Ra'avad always requires three on the road. One can with resolve this. The Shulchan Aruch did not bring R. Yerucham's leniency, therefore one should not rely on it.

Drishah (12): When there are three, we are not concerned lest two men need to move their bowels at the same time. Likewise, if they plan to stay awake, we are not concerned lest two sleep at the same time. If they plan to sleep, it is forbidden. Therefore, a young boy or girl cannot be a Shomer (a third to evade the Isur of Yichud, for he is prone to sleep). People rely on this, even to go with a Nochri wagon-driver! It seems that they rely on the Rif. The Rif omitted the distinction between the city and the road because he is not concerned (for Yichud when one goes to move his bowels). She will go with her child when he goes to do his needs. If the man tries to rape her at night, she will wake the child. However, the Ra'avad did not rely on this. Also, perhaps he will entice her at night! It seems that only the Ra'avad is concerned for this. The others are concerned only for rape, and that she will conceal the matter due to shame. Therefore, one Shomer suffices.