Mishnah Mikvaos 3:3: If there is a pit full of drawn waters, and a channel [of rainwater] is entering into to and leaving from it, it is forever in its invalid state until it is calculated that there no longer remain [in the pit] three log [or more, of drawn waters] from the original amount. Two individuals who were dropping, this one a log and a half and that one a log and a half, into a mikveh; or one who wrings out his clothing and it drops [water into the mikveh] from many areas [of the clothing]; or one who pours out [water] from a tzirtzur [a stone vessel with a netted opening] and it drops from many areas [of the vessel]; Rabbi Akiva considers these [such cases] valid, and the Sages consider them invalid. Rabbi Akiva said, "They did not say 'if they drop [three log of drawn waters, it is rendered invalid],' but rather 'if it drops.' [implying that it is only invalid if the water was dropped in from only one source]" They said to him, "They said neither this nor that. Rather [they said,] 'one that three log fell into it [is rendered invalid, and there is no distinction between dropping from one or multiple sources].'"

בּוֹר שֶׁהוּא מָלֵא מַיִם שְׁאוּבִין וְהָאַמָּה נִכְנֶסֶת לוֹ וְיוֹצְאָה מִמֶּנּוּ, לְעוֹלָם הוּא בִפְסוּלוֹ, עַד שֶׁיִּתְחַשֵּׁב שֶׁלֹּא נִשְׁתַּיֵּר מִן הָרִאשׁוֹנִים שְׁלשָׁה לֻגִּין. שְׁנַיִם שֶׁהָיוּ מַטִּילִין לַמִּקְוֶה, זֶה לֹג וּמֶחֱצָה וָזֶה לֹג וּמֶחֱצָה, הַסּוֹחֵט אֶת כְּסוּתוֹ וּמַטִּיל מִמְּקוֹמוֹת הַרְבֵּה, וְהַמְּעָרֶה מִן הַצַּרְצוּר וּמַטִּיל מִמְּקוֹמוֹת הַרְבֵּה, רַבִּי עֲקִיבָא מַכְשִׁיר, וַחֲכָמִים פּוֹסְלִין. אָמַר רַבִּי עֲקִיבָא, לֹא אָמְרוּ מַטִּילִין, אֶלָּא מַטִּיל. אָמְרוּ לוֹ, לֹא כָךְ וְלֹא כָךְ אָמְרוּ, אֶלָּא שֶׁנָּפְלוּ לוֹ שְׁלשָׁה לֻגִּין:

Mishnah Mikvaos 3:4: If [three log of drawn waters fell into a mikveh] from one vessel, from two, or from three, they join together [to add up to the required measure to render the mikveh invalid]; but from four [vessels], they do not join together. A ba'al keri [a male who has had a seminal emission but has yet to purify himself by immersion in a mikveh] who is ill, if nine kav [a specific volume measurement, equal to four log] fell on him [which the Sages established to be sufficient to purify him in such a case when he is ill and therefore cannot immerse], or a pure individual who three log of drawn waters fell on his head and the majority of his body [which the Sages established renders one impure], if [they fell on him] from one vessel, or from two, or from three, they join together [to add up to the minimum measure required to render them either pure or impure, respectively]; if from four, they do not join together [to add up to the required measure to have an effect]. Regarding what were these words said? Regarding a time when the second [vessel] began [pouring] before the first had stopped. And regarding what were these words said [i.e. that three log poured from four vessels, or not poured in close succession, do not join together to render a mikveh invalid]? Regarding a time when one did not intend to increase [the waters of the mikveh]; but if one did intended to increase, even [only by] a kortov every year, they do join together to three log [rendering the mikveh invalid].

מִכְּלִי אֶחָד, מִשְּׁנַיִם וּמִשְּׁלשָׁה, מִצְטָרְפִין. וּמֵאַרְבָּעָה, אֵין מִצְטָרְפִין. בַּעַל קֶרִי הַחוֹלֶה שֶׁנָּפְלוּ עָלָיו תִּשְׁעָה קַבִּין מַיִם, וְטָהוֹר שֶׁנָּפְלוּ עַל רֹאשׁוֹ וְעַל רֻבּוֹ שְׁלשָׁה לֻגִּין מַיִם שְׁאוּבִין, מִכְּלִי אֶחָד, מִשְּׁנַיִם וּמִשְּׁלשָׁה, מִצְטָרְפִין. מֵאַרְבָּעָה, אֵין מִצְטָרְפִין. בַּמֶּה דְבָרִים אֲמוּרִים. בִּזְמַן שֶׁהִתְחִיל הַשֵּׁנִי עַד שֶׁלֹּא פָסַק הָרִאשׁוֹן. וּבַמֶּה דְבָרִים אֲמוּרִים. בִּזְמַן שֶׁלֹּא נִתְכַּוֵּן לְרַבּוֹת. אֲבָל נִתְכַּוֵּן לְרַבּוֹת, אֲפִלּוּ קֹרְטוֹב בְּכָל הַשָּׁנָה, מִצְטָרְפִין לִשְׁלשָׁה לֻגִּין: