Gemara

(Rav): If three sisters fell to Yibum to two brothers, one sister does Chalitzah with one brother, another does Chalitzah with the other brother, and the third sister needs Chalitzah from both of them.

(Rabah): Since Rav requires a sister to do Chalitzah with both, he must hold that Yesh Zikah, and the Chalitzah of the third sister is Pesulah (Yibum was forbidden), and a Chalitzah Pesulah requires Chazarah (the Choletzes must do Chalitzah with all the brothers).

Even though Rav (17b) taught that Ein Zikah, here he teaches according to the opinion that Yesh Zikah.

(Shmuel): One brother (Reuven) may do Chalitzah to all of them.

Question: Regarding sisters with Tzaros who fell to Yibum, Shmuel requires a proper Chalitzah. Here, Shimon could do a proper Chalitzah (to the second Yevamah). Why is it enough for Reuven to do Chalitzah Pesulah?!

Answer #1: Shmuel teaches that one does Chalitzah to all of them, i.e. to the first and third.

Objection: But he said 'to all of them'!

Answer: Since he does Chalitzah to the majority, this is called all of them.

Answer #2: Shmuel requires proper Chalitzah only to exempt the Tzarah, but the Choletzes herself is exempted in any case;

(Shmuel): (When sisters with Tzaros fell to one Yavam,) if he did Chalitzah to the sisters, the Tzaros are not exempted. If he did Chalitzah to the Tzaros, the sisters are exempted;

Question: Through Zikah, each sister is like Achos Ishto. Therefore Chalitzah to the sisters is Pesulah, so it does not exempt the Tzaros. Also Chalitzah to the Tzaros should not exempt the sisters, because through Zikah each is the Tzarah of Achos Ishto!

Answer #1: Even though Shmuel (18b) taught that Yesh Zikah, here he teaches according to the opinion that Ein Zikah.

Question: If so, when he does Chalitzah to the sisters (Leah and then Rachel), why aren't the Tzaros exempted?

Granted, Rachel's Tzarah is not exempted. Since he already did Chalitzah to Leah, Rachel's Chalitzah is Pesulah. But Leah's Chalitzah was valid, so her Tzarah should be exempted!

Answer: Indeed, she is exempted! Shmuel said 'the Tzaros (plural) are not exempted' to teach that in all such cases, Rachel's Tzarah is not exempted.

Question: If so ('does not exempt' means that only one is exempted), when he says that Chalitzah of the Tzaros (e.g. Leah's and then Rachel's) exempts the sisters, he must mean both sisters. But the Chalitzah of Rachel's Tzarah is Pesulah, so it does not exempt Rachel!

(Mishnah): Tzaras Krovas Chalutzaso is forbidden.

Answer: Shmuel taught that if he started doing Chalitzah with (one of) the sisters, he does not finish with the Tzaros (it is not enough to do Chalitzah to her sister's Tzarah), because Tzaras Krovas Chalutzaso is forbidden;

If he started doing Chalitzah with (one of) the Tzaros (e.g. Leah's), he may finish even with the sisters (i.e. Rachel), because Krovas Tzaras Chalutzaso is permitted.

Answer #2 (Rav Ashi): We need not change Shmuel's words. (When he does Chalitzah to the sisters, no Tzarah is exempted. Even though Yesh Zikah,) the Chalitzah of Rachel's Tzarah is considered a proper Chalitzah, since Zikah is not so strong to make the Tzarah like Ervah. (Chachamim decreed only about Achos Zekukaso.)

Support (Beraisa): If he did Chalitzah to the sisters, the Tzaros are not exempted.

Inference: Chalitzah to the Tzaros would exempt the sisters.

Suggestion: This is because Yesh Zikah, but Zikah is not so strong to make the Tzarah like Ervah.

Rejection (Rav Aba bar Mamal): The Beraisa is like Beis Shamai, who permit Tzaras Ervah. Mid'Oraisa, even Yibum is allowed. The Beraisa is like R. Yochanan ben Nuri, who suggested enacting that all Tzaros do Chalitzah but not Yibum.

(Rav Nachman bar Yitzchak): Even though they were not able to enact this in time, later, it was enacted.

27b - Question: If one Yevamah received a Get and another received a Ma'amar, which Chalitzah is better?

Answer (Rav Ashi - Mishnah): R. Gamliel agrees that a Get takes effect after a Ma'amar, and a Ma'amar after a Get. This shows that they are equal.

Rishonim

Rif: If three sisters fell to Yibum to two brothers, Rav says that one sister does Chalitzah with one brother, another does Chalitzah with the other, and the third needs Chalitzah from both. Shmuel allows one brother to do Chalitzah to all of them. He requires a proper Chalitzah only to exempt the Tzarah, but the Choletzes herself is exempted in any case. Shmuel says that when sisters with Tzaros fell to one brother, Chalitzah to the sisters does not exempt the Tzaros. Chalitzah to the Tzaros exempts the sisters, because Zikah is not so strong to make the Tzarah like Ervah. Our Rebbi'im rule like Shmuel.

Hasagos ha'Ra'avad (brought in Sefer ha'Zechus): The Rif did not need to rule like Shmuel regarding the second law (Chalitzah to the sisters or the Tzaros), for no one argues. Rather, he rules like Shmuel's first law, that one brother may do Chalitzah to all of them, unlike Rav.

Rebuttal (Sefer ha'Zechus): The Kelal is that the Halachah follows Rav against Shmuel (in Isurim, so the Rif did not need to rule like Rav explicitly). One might have thought that just like one who does Chalitzah Pesulah requires Chalitzah from all the brothers (like Rav), also all the Tzaros must do Chalitzah. Therefore, he needed to rule like the version of Shmuel that says that Chalitzah to the Tzaros exempts the sisters.

Nimukei Yosef (DH Gemara): Even though the Halachah follows Rav against Shmuel in Isurim, Rav himself says that the Chalitzah is Kesherah because Ein Zikah. He is stringent according to Shmuel; since Shmuel himself permits, the Halachah permits like Shmuel.

(Rosh 3:1): The Rif brings only the second answer why Shmuel allows one brother to do Chalitzah to all of them. However, the Ri (in Tosfos 27a DH Mai) proves that the first answer below (that Chalitzah of the sisters does not exempt the Tzaros', i.e. Rachel's Tzarah) holds like this second answer 'above'. The conclusion below is like Rav Ashi, who says that Shmuel discusses starting and ending with the sisters or Tzaros, and he holds like the first answer above.

Rebuttal (Yam Shel Shlomo 3:2 and Lechem Mishneh Hilchos Yibum 5:10): The Ri said only that the first answer below must be like the second answer above. Rav Ashi could also agree with the second answer above! The Rosh himself agrees with the Rif that the Halachah normally follows the second version! The Rambam rules like the second answer above, and like Rav Ashi.

Defense (Korban Nesan'el 1): Tosfos (27a DH Acharav) says that R. Aba bar Mamal established the Beraisa like Beis Shamai, even though this is difficult, because it is unreasonable to say that Zikah does not make the Tzarah like Ervah (since also Tzaras Ervah is Chayavei Kerisos). The Rosh infers that Rav Ashi was forced to say that Zikah does not make the Tzarah like Ervah because he holds like the first answer above and disagrees with the second answer.

Rambam (11): If one of the Tzaros did a proper Chalitzah, this exempts all the Tzaros and removes the Zikah.

Rambam (12): If Leah did an inferior Chalitzah, she is permitted to marry a stranger, but her Tzarah is forbidden until she does Chalitzah, or until all the brothers do Chalitzah to Leah.

Question (Magid Mishneh): The Rambam says that the Choletzes herself is exempted, and that if she did Chalitzah with all the brothers she exempts her Tzarah. I do not know his source.

Answer (Kesef Mishneh): The Tur says that the Rambam rules like Shmuel. Seemingly, the Rambam rules like Rav, who says that Chalitzah Pesulah requires Chazarah! However, the Rambam says that Chalitzah Pesulah exempt the Choletzes herself, like Shmuel. He requires Chazarah only to exempt the Tzarah.

Poskim

Shulchan Aruch (EH 170:5): If a Yevamah received a Get, according to the opinion that Chalitzah Pesulah requires Chazarah, when there are two Yevamim and one Yevamah she needs Chalitzah from each brother. Some say that similarly when there are two Yevamos and one brother, each Yevamah needs Chalitzah. According to the opinion that Chalitzah Pesulah does not require Chazarah, even when there are two Yevamim and one Yevamah one Chalitzah suffices.

Rema: The Rosh agrees with this. We require Chazarah when she was totally Zekukah and became Pesulah, i.e. the Zikah became weaker. If the Chalitzah was Pesulah from the beginning, Chazarah is not needed.

Gra (13,14): Regarding Chalitzos that are equally Pasul, Rav requires Chazarah (Chalitzah from every brother), but Chalitzah of one exempts the Tzarah. According to the latter opinion (of Shmuel's argument with Rav), Shmuel holds oppositely. Inferior Chalitzah exempts the Yevamah herself, but not the Tzarah.

Shulchan Aruch (19): If many Yevamos fell from one brother, and Leah had an inferior Chalitzah, she is permitted but her Tzarah is forbidden until she does Chalitzah, or until all the brothers do Chalitzah with Leah. This is the Rambam's opinion (like it says explicitly in the Tur), that inferior Chalitzah does not exempt until she does Chalitzah with all the brothers, or until all the Tzaros do Chalitzah.

Beis Yosef (DH u'Vasar): Tosfos (27b DH Eisivei) says that the Gemara challenged R. Yochanan but not Rav because R. Yochanan holds like Shmuel. Since the Halachah follows R. Yochanan, this shows that the Halachah follows Shmuel.