Reference for Mikvaot 6:3
שְׁלשָׁה מִקְוָאוֹת, בָּזֶה עֶשְׂרִים סְאָה, וּבָזֶה עֶשְׂרִים סְאָה, וּבָזֶה עֶשְׂרִים סְאָה מַיִם שְׁאוּבִין, וְהַשָּׁאוּב מִן הַצַּד, וְיָרְדוּ שְׁלשָׁה וְטָבְלוּ בָהֶן וְנִתְעָרְבוּ, הַמִּקְוָאוֹת טְהוֹרִין וְהַטּוֹבְלִים טְהוֹרִים. הָיָה הַשָּׁאוּב בָּאֶמְצַע, וְיָרְדוּ שְׁלשָׁה וְטָבְלוּ בָהֶם וְנִתְעָרְבוּ, הַמִּקְוָאוֹת כְּמוֹת שֶׁהָיוּ, וְהַטּוֹבְלִים כְּמוֹת שֶׁהָיוּ:
Regarding three <i>mikvaot</i>, one containing twenty <i>se'ah</i> [a specific unit of volume], one containing twenty <i>se'ah</i>, and one containining twenty <i>se'ah</i> of drawn water [while the other two contain valid water], and the drawn [water <i>mikvah</i>] was to the side, if three people descended and immersed in them [at once, one in each], and they [caused the waters to overflow such that the three <i>mikvaot</i>] were mixed, the <i>mikvaot</i> are pure and the immersers are pure. If the drawn [water <i>mikvah</i>] was in the middle, and three people descended and immersed, and they were mixed, the <i>mikvaot</i> are as they were [before the people immersed], and the immersers remain as they were [i.e. impure].