Reference for Mikvaot 3:4
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:
If [three <i>log</i> of drawn waters fell into a <i>mikveh</i>] from one vessel, from two, or from three, they join together [to add up to the required measure to render the <i>mikveh</i> invalid]; but from four [vessels], they do not join together. A <i>ba'al keri</i> [a male who has had a seminal emission but has yet to purify himself by immersion in a <i>mikveh</i>] who is ill, if nine <i>kav</i> [a specific volume measurement, equal to four <i>log</i>] 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 <i>log</i> 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 <i>log</i> poured from four vessels, or not poured in close succession, do not join together to render a <i>mikveh</i> invalid]? Regarding a time when one did not intend to increase [the waters of the <i>mikveh</i>]; but if one did intended to increase, even [only by] a <i>kortov</i> every year, they do join together to three <i>log</i> [rendering the <i>mikveh</i> invalid].