Pesachim, Chapter 5, Mishnah 7. In Mishnah 5 we learned that the slaughtering of the Pesach offering was carried out in three groups. Our Mishnah continues and describes the order in which the groups entered and left, and the recitation of Hallel that accompanied the service.
The Order in Which the Groups Entered:
"Yatza kat rishonah, nichnesah kat sheniyah" - once the first group had completed its service and left, the second group entered.
"Yatza sheniyah, nichnesah shelishis" - and likewise, after the second group left, the third group entered.
"Kema'aseh harishonah kach ma'aseh hasheniyah vehashelishis" - all the procedures described for the first group were repeated in the very same order by the second and the third.
The Recitation of Hallel:
While the Pesach offering was being slaughtered and prepared, "kar'u es haHallel" - the Levi'im sang Hallel, the same Hallel we recite on the night of Pesach, during the performance of the offering.
"Im gamru shanu, ve'im shanu shileshu" - if the Levi'im finished Hallel and there were still Pesach offerings left to bring, they would repeat it a second time; and if even after the second time offerings remained, they would recite it a third time.
"Af al pi shelo shileshu miyemeihem" - this is the rule, but in practice they never needed to recite it a third time, since the kohanim worked with great speed and managed to complete the entire service within two recitations of Hallel.
The Words of Rabbi Yehudah:
Rabbi Yehudah adds an interesting fact: "Miyemeihem shel kat shelishis lo higi'u le'Ahavti ki yishma Hashem, mipnei she'ameha mu'atin" - in all the days of the third group, the Levi'im never even reached the chapter of "Ahavti ki yishma Hashem" in Hallel. Not only did they not repeat it a second or a third time, they did not even finish the first recitation. The reason: the people of the third group were few, because everyone hurried to be counted among the first two groups, and therefore by the time the Levi'im reached "Ahavti ki yishma Hashem" its service was already completed.
In summary: in this Mishnah we learned the order in which the three groups entered and left, each one repeating the procedure of the one before it; the recitation of Hallel during the service, which was repeated as needed and never had to be said three times; and the words of Rabbi Yehudah, that in the third group, because of its small numbers, the Levi'im never even reached "Ahavti ki yishma Hashem".