Zavim, Chapter 1, Mishnah 3. The opening mishnayos of this masechta lay out the levels of tumah that come upon a man who experiences a zivah flow. The number of re'iyos (separate experiences of the flow) determines everything: how severe his tumah is, and whether he is obligated in a korban at the end of his purification. This mishnah brings us to the top of that ladder.
The cases of the mishnah
The mishnah lists several combinations that all arrive at the same result:
"Ra'ah achas hayom ushtayim lemachar": he had one re'iyah on a given day, and on the following day he had two separate re'iyos.
"Shtayim hayom ve'achas lemachar": or the reverse, two re'iyos on the first day and a single one on the day after.
"Shalosh lishlosha yamim": or three re'iyos spread over three days, one on each day.
"O lishlosha leilos": or three re'iyos over the course of three nights.
The ruling
"Harei zeh zav gamur": in every one of these cases the man is a full-fledged zav. Since three re'iyos have accumulated, he is not merely tamei; he is also obligated to bring a korban when he completes his taharah. The mishnah's point is that the three re'iyos need not be packed into a single stretch of time. Whether they come two and one, one and two, or one on each of three consecutive days or nights, the total is what counts.
The condition of consecutive days
All of this assumes that the days follow one another without a break. Where a clear day intervenes, with no flow at all, we return to the dispute recorded earlier in the perek. According to Beis Hillel, the re'iyos on either side of that empty day do not join together to create this tumah, while Beis Shammai hold otherwise.
So the third mishnah gives us the finished picture of the zav gamur: three re'iyos, arranged in whichever pattern over consecutive days or nights, produce the complete status, with all its tumah and with the obligation of a korban.