I was looking at documentation of a mikvah being constructed, and it confused me. In addition to the immersion pool, this mikvah had three additional chambers. From what I have gathered, they appear to be 1) a bor hashakah, 2) a bor zeriah, and 3) a bor tachton.

If I understand correctly, the purpose of each of these chambers is the same: to provide a way for the tap water to be considered kosher for the mikvah. What I don't understand is why three different kinds are present in one mikvah.

How common is it for a modern mikvah to have all three chambers as this one did? What is the most common scenario? Is it necessary to have all three, or what is the benefit in doing so?