Mishnah Mikvaos 2:5: Regarding a mikveh that has three furrows containing drawn waters [along its sloping side, such that each furrow is above the next], with one log in each, if it is known that forty se'ah of valid water fell in before they reached the third furrow, it is valid; and if not, it is invalid. Rabbi Shimon considers it valid because it is like a mikveh next to a mikveh.

מִקְוֶה שֶׁיֶּשׁ בּוֹ שָׁלשׁ גֻּמּוֹת שֶׁל מַיִם שְׁאוּבִין שֶׁל לֹג לֹג, אִם יָדוּעַ שֶׁנָּפַל לְתוֹכוֹ אַרְבָּעִים סְאִין מַיִם כְּשֵׁרִין עַד שֶׁלֹּא הִגִּיעוּ לַגֻּמָּא הַשְּׁלִישִׁית, כָּשֵׁר. וְאִם לָאו, פָּסוּל. וְרַבִּי שִׁמְעוֹן מַכְשִׁיר, מִפְּנֵי שֶׁהוּא כְמִקְוֶה סָמוּךְ לְמִקְוֶה:

Mishnah Mikvaos 2:6: One who removes the plaster [from the floor of mikveh] to the sides, and three log [of water] flowed from it [from the plaster], it is valid. If one removed [the plaster from the waters of the mikveh] and three log flowed out from it, it is invalid [since they are considered drawn waters]. And Rabbi Shimon considers it valid it because one did not intend to draw [the waters in such a case].

הַמְסַנֵּק אֶת הַטִּיט לַצְּדָדִין וּמָשְׁכוּ מִמֶּנּוּ שְׁלשָׁה לֻגִּין, כָּשֵׁר. הָיָה תוֹלֵשׁ וּמָשְׁכוּ מִמֶּנּוּ שְׁלשָׁה לֻגִּין, פָּסוּל. וְרַבִּי שִׁמְעוֹן מַכְשִׁיר, מִפְּנֵי שֶׁלֹּא נִתְכַּוֵּן לִשְׁאֹב: