Gemara

3b (Rav): (If three Lugim (a Lug is about a half liter) of Mayim She'uvim, i.e. water that was in a vessel, falls into an incomplete Mikveh, the Mikveh is Pasul even after it has a full Shi'ur.) If a Kortov (a 64th of a Lug) of wine fell into three Lugim of Mayim She'uvim and gave it the appearance of wine, it (is considered like wine, so it) does not disqualify a Mikveh (if it fell in).

Question (Rav Kahana): Why is it different than dye water?

(Mishnah - R. Yosi): If three Lugim of dye water fell into a Mikveh, they disqualify it.

Answer (Rava): Since it is called dye water, the law of Mayim She'uvim applies. In Rav Yehudah's case, it is called mixed (diluted) wine, so the law of Mayim She'uvim does not apply.

Question (R. Chiya's Beraisa): A case (like Rav discusses) occurred, and they disqualified the Mikveh.

Answer (Rava): Rav holds like R. Yochanan ben Nuri, and R. Chiya's Beraisa holds like Chachamim.

(Mishnah): If a Kortov of wine fell into water, and the volume of the mixture is three Lugim, and it looks like wine, the mixture does not disqualify a Mikveh. The same applies if a Kortov of milk fell into water, and there are three Lugim in the mixture, and it looks like water.

R. Yochanan ben Nuri says, it all depends on the appearance (if it looks like water, it disqualifies a Mikveh).

Question: Rav Papa was unsure if Rav's text of the Reisha of the Mishnah said that the total volume is three Lugim, but if there were three Lugim of water alone, Chachamim disqualify the Mikveh? If so, Rav holds like R. Yochanan (ben Nuri). Or, perhaps Rav's text say that the volume of the water is three Lugim, and R. Yochanan argues only in the Seifa? If so, Rav's law is like everyone.

Answer: Rav Papa was unsure. Rava was sure (Rav holds like R. Yochanan).

Abaye: Rav Yosef taught us that Rav's text says that there are three Lugim of water, and Rav's law is like everyone!

Temurah 12b (Beraisa - Yosef ben Choni): If three Lugim were poured into a Mikvah from at most three Kelim, they disqualify the Mikvah. If they came from four or more Kelim, they do not disqualify it.

Rishonim

Rif (Shevuos 5b): Three Lugim of She'uvim disqualify an incomplete Mikveh. One may add She'uvim to a complete Mikveh, or to an incomplete Mikveh through Hamshachah (he pours water on the ground, and it flows into the Mikveh.

Rambam (Hilchos Mikva'os 5:1): If three Lugim of Mayim She'uvim fell into a Mikveh from one or two or three Kelim, they join. Four Kelim do not join. This is if he did not intend to increase. If he intended to increase the water of the Mikveh, even if a Dinar (about four grams) of water fell each year, it joins to three Lugim, whether the She'uvim fell before or after or together with the Kosher water. Since they fell before there were 40 Sa'im, it is Pasul, and all the water becomes She'uvim.

Rambam (7:10): If a Dinar of wine fell into three Lugim of Mayim She'uvim, and the mixture looks like wine, it does not disqualify a Mikveh if it falls in, unless it changed the Mikveh's appearance.

Rambam (11): If a Dinar of milk or fruit juice fell into three Lugim less a Dinar of Mayim She'uvim, and the mixture looks like water, it does not disqualify a Mikveh, unless there were three Lugim of water alone.

Rosh (Hilchos Mikva'os (b'Sof Hilchos Nidah) 4): If three Lugim fell from two or three Kelim, they join. This is when the water starting to flow from the second Keli before it ceased to flow from the first. Four do not join, unless he intended from the beginning to pour in all three Lugim. Then, they disqualify even if he poured in slowly from many Kelim.

Rosh (20): Chachamim require a Shi'ur and appearance (of water). Therefore, it does not disqualify in the Reisha or Seifa. R. Yochanan ben Nuri argues in the Seifa. The Halachah follows Chachamim.

Poskim

Shulchan Aruch (YD 201:15): Even if the three Lugim fell from two or three Kelim, a little from each, they join. This is when the water starting to flow from the second Keli before the water ceased to flow from the first. If there was a break after the first ceased to flow, it does not disqualify. Three Kelim join, but four do not, i.e. if he did not intend from the beginning to pour all three Lugim in.

Beis Yosef (DH v'Da): Rashi (Temurah 12b DH b'Veis) says that three Lugim from two or three Kelim disqualify only if at least a Lug fell from each Keli. If two Lugim fell from one Keli, and one Lug from the other two Kelim combined, they do not disqualify it, for less than one Log fell from each Kli.

Gra (50): Three Lugim from one Keli disqualify in any case (even if the flow stopped in the middle).

Shach (48): Four Kelim do not join even if they fell in at once, for there is less than a Log in each Keli.

Shulchan Aruch (ibid.): If he intended from the beginning to pour in all three Lugim, they disqualify the Mikveh even if he poured in slowly from many Kelim.

Shach (49): Mikva'os 3:4 says that if he intended Lerabos, they join. The Rambam, Bartenura, Beis Yosef and Tosfos Yom Tov explain that if he intended to add Mayim She'uvim to the Mikveh so it will not become deficient due to water lost through immersions, then they join. The Rashba says that if he intended to add them to the Mikveh, they join even from 10 Kelim even over a long time.

Shulchan Aruch (23): The three Lugim disqualify only if they were complete (the full Shi'ur) and they had the appearance of water. Therefore, if wine fell into three Lugim of water, and the mixture looks like wine, it does not disqualify a Mikveh if it falls in. Similarly, if milk fell into any amount less than three Lugim of water, and completed the Shi'ur of three Lugim, if it fell into a Mikveh it does not disqualify it.

Taz (33): The Reisha discusses (red) wine, for it is more prone to change the appearance than milk is.

Gra (58): We rule like the first Tana and Rav, according to the conclusion in Makos

Beis Yosef (DH u'Mah): The Rosh explicitly holds that the Mikveh is Kosher only if what fell in did not change the appearance of the Mikveh. This is obvious.

Bach (29): I did not see the Rosh say so particularly about a deficient Mikveh. If wine fell even into a full Mikveh and now it looks like wine, it is Pasul!

Hagahos Tur ha'Shalem (103) and Hagahos Shulchan Aruch (Friedman edition) 52: Perhaps the text of the Beis Yosef should say 'the Rambam explicitly holds that...'

Note: The Shach cites the Beis Yosef like our text, and adds 'this is obvious, for change of appearance disqualifies even a complete Mikveh.'

Machatzis ha'Shekel (60 DH Im): Even if the Mikveh does not look like wine, since the appearance changed, it is Pasul.