Reference for Mikvaot 3:3
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:
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 <i>log</i> [or more, of drawn waters] from the original amount. Two individuals who were dropping, this one a <i>log</i> and a half and that one a <i>log</i> and a half, into a <i>mikveh</i>; or one who wrings out his clothing and it drops [water into the <i>mikveh</i>] 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 <i>log</i> 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 <i>log</i> fell into it [is rendered invalid, and there is no distinction between dropping from one or multiple sources].'"