Negaim, Chapter 14, Mishnah 4. This chapter tracks the purification of a metzora stage by stage: the birds, the sprinkling, the release of the living bird, the first shaving with the immersion of his garments and his body, and the seven days during which he is forbidden to his wife. Our mishnah takes us to the seventh day, to the second shaving, and to the three distinct stages of taharah that a metzora passes through before he may eat kodshim.
The second shaving
"Bayom hashvi'i megalei'ach tiglachas shniyah k'siglachas rishonah," on the seventh day he shaves a second time, in the same manner as the first shaving. Once again a razor is passed over his body exactly as it was the first time.
"Chibeis b'godov v'toval," he immerses his garments and immerses himself. Here the immersion of the garments accomplishes something new. Since the first shaving he had been conveying tumah like a sheretz, so his garments had become a rishon l'tumah through his touch. Now they are immersed in a mikveh and they emerge entirely tohor. He too goes to the mikveh, and with that, "tohor mitamei k'sheretz," he no longer conveys tumah on the level of a sheretz. "V'harei hu tevul yom," and now he has the status of a tevul yom.
What a tevul yom is
He immersed during the daytime of the seventh day, and until sunset he carries the status of a tevul yom: one who has immersed but must still wait for ha'arev shemesh, for the sun to set, before he may eat terumah. He may already eat ma'aser, as the mishnah goes on to say, but not terumah.
The Rav writes: "tohor mitamei k'sheretz u'poseil terumah v'tu lo," he no longer conveys tumah like a sheretz, and he can only invalidate terumah, nothing more. A tevul yom, as you will learn in its own place in Maseches Tevul Yom, is an interesting status: something like a sheni l'tumah, with certain leniencies and certain stringencies. A sheni renders what it touches a shlishi. In chullin there is no such thing as a shlishi, but in terumah there is: terumah that becomes a shlishi may no longer be eaten by a kohen, yet if that terumah then touches other terumah, the second terumah is unaffected. It is treated as tamei with regard to eating, but it does not generate a further level of tumah, a revi'i. So the tevul yom's reach extends only as far as making terumah a shlishi, disqualifying it from being eaten, without transmitting tumah any further. That is the Rav's "v'tu lo," and no further than that.
Ma'aser, terumah, kodshim
Now the mishnah charts out the sequence. "Tevul yom ochel b'ma'aser," while he stands in the status of a tevul yom, that is, from the moment he rises out of his immersion on the seventh day, ma'aser is permitted to him. "He'eriv shimsho, ochel b'serumah," once the sun has gone down and night has arrived, tzeis hakochavim, terumah becomes permitted. "Heivi kaparaso, ochel b'kodshim," and after his offerings of atonement have been presented the following day, kodshim too become open to him.
Until he brings those korbonos he remains mechussar kaparah, lacking his atonement, and even at night, when terumah has become permitted to him, kodshim are still closed to him. Terumah is not kodshei mizbei'ach, offerings of the altar, which is why it becomes permitted at nightfall while kodshim must wait for the korban. On the level of Torah law, nothing more than what the mishnah lists is required. There is a discussion in Tosfos, in Chagigah among other places, concerning a rabbinic requirement of an additional immersion connected with the bringing of his korbonos, some placing it after and some before, but that is a takanah d'Rabonon; mid'Oraisa he eats kodshim once his kaparah has been brought, with no further tevilos.
"Nimtz'u sholosh taharos b'metzora"
The mishnah's summary reads: "nimtz'u sholosh taharos b'metzora," a metzora turns out to pass through three separate points of taharah. The first arrives with his immersion at the close of the seven days, which leaves him a tevul yom, permitted in ma'aser while terumah is still withheld from him. The second arrives with the appearance of the stars that night, when terumah becomes his to eat. The third comes with the korbonos of the next day, and from that point onward kodshim are permitted to him as well.
"V'chein sholosh taharos b'yoledes," and the same three stages apply to a yoledes, a woman who has given birth. She too immerses, becomes a tvulas yom who may eat ma'aser but not terumah until night, and on the day she brings her korbonos she may eat kodshim.
The Rav's addition: zav and zavah
The Rav asks why the mishnah mentions specifically a yoledes, since "hu hadin b'zav ba'al shalosh re'iyos," the identical law applies to a zav who has had three emissions. A man who experiences three re'iyos of that particular kind of flux, whether in one day or on three separate days, is a zav who must bring a korban. He counts his days of purification, immerses on the seventh day and is then a tevul yom who may eat ma'aser, at nightfall he may eat terumah (assuming, of course, that he is a kohen), and the next day, once he has brought his offerings, he may eat kodshim. Note that this applies specifically to a zav of three re'iyos; one who had only two re'iyos brings no korban and needs only immersion.
A zavah stands in the same position, that is, a zavah gedolah, one who is obligated in korbonos. Once a woman's days of niddah have come to a close, there is a stretch of eleven days in which niddah status cannot begin again, and it is precisely in that stretch, lying between one niddah period and the next, that zavah status can take hold. Should she see blood on three separate days within those eleven, she has the status of a zavah gedolah and owes a korban. She then counts seven clean days, and upon immersing at the end of the seventh she is permitted in ma'aser; when the stars come out terumah is permitted to her, provided that she is a woman with a portion in terumah (an unmarried bas kohen, a woman married to a kohen, or a bas kohen who was widowed childless from a Yisroel husband and has returned to eat at her father's table); and once her korban has been offered, kodshim are permitted to her too.
For all of them, then, the pattern is identical: "toval v'oleh, ochel b'ma'aser," they immerse and come up and may eat ma'aser immediately; "he'eriv shimsho, ochel b'serumah," at tzeis hakochavim terumah becomes permitted, for those to whom terumah applies; "heivi kaparaso, ochel b'kodshim," once the korban has been brought, kodshim too. Three taharos in each case.
The long-term tvulas yom
In the case of a yoledes it is worth noting that the seven days (for a male child) or fourteen days (for a female child) are only the beginning. After the days of tumah come the days of dam tohar: thirty-three days for a male and sixty-six for a female. Only at the end of that period does she bring her korbonos. Throughout that time, until her korban is brought, she carries the status the Gemara calls a tvulas yom aroch, an extended tvulas yom: forty days in the case of a male child and eighty days in the case of a female child. The three stages the mishnah speaks of therefore refer to the concluding day of her count, when she immerses, waits for nightfall, and then brings her offering the following day.
Why the mishnah singles out the yoledes
The Rav explains why the mishnah chose to mention a yoledes rather than a zav or a zavah: "mishum d'yoledes k'siv b'hedya," because in the case of a yoledes the Torah states it explicitly. The posuk says "v'chiper oleha hakohen v'toheiroh," the kohen shall atone for her and she shall be tohorah, and that taharah is understood as taharah for eating kodshim. That posuk appears in Parshas Shemini, in the Torah's treatment of the yoledes, whereas the zav and the zavah are set out at the end of Parshas Metzora, where no such explicit statement is made about eating kodshim. Their law is learned instead through a binyan av or a hekesh from the yoledes. Since the yoledes is the case stated openly and serves as the model for the others, the mishnah pairs her with the metzora: "nimtz'u sholosh taharos b'metzora, v'chein sholosh taharos b'yoledes."