Mikvaot, Chapter Six, Mishnah Three
1) Three mikvehs, two of which held twenty seahs [of valid water] and the third held twenty seahs of drawn water, and that holding drawn water was at the side:
a) If three persons went down and immersed themselves in them and [the water of the three mikvehs] joined, the mikvehs are clean and they that immersed themselves become clean.
2) If the one holding the drawn water was in the middle and three persons went down and immersed themselves in them and [the water of the three mikvehs] joined, the mikvehs continue as they were before and they that immersed themselves are as they were before.
Section one: When the three people go down into the three mikvaot the water level rises and the water from each mikveh is mixed with the others. Since the mikveh with the invalid drawn water is at the side, this is a case where the two mikvaot with the valid water are mixed together first and then they together validate the drawn water from the third mikveh.
Section two: If the mikveh with the invalid water is in the middle, then none of the mikvaot are rendered valid because the drawn water invalidates them both. However, the water is not invalidated because we don’t know for sure that three logs of drawn water have gone in to the other mikvaot. If he were to subsequently fill one of these mikvaot, it would become valid. The people that immersed remain impure.