Tosefta for Yevamot 118:7

A girl whose time to see [a menstrual flow] had not yet come, and she saw a her first menstrual flow (lit., "saw her first seeing") and her second [menstrual flow], then by her third time she may impart impurity for a twenty-four hour period. If she skipped two cycles (alt, שלש עונות ("three cycles"), see Ohr HaGanuz and Nidd. 9b:3) and then saw [a menstrual flow], her time is sufficient [to be subject to the laws of niddah]. Or if she arrived at her time to see [a menstrual flow] and she saw her first menstrual flow, and her second, she imparts impurity for twenty-four hours, and on the third [menstrual flow] her time is sufficient. if she skipped three cycles and then she saw [a menstrual flow], she imparts impurity for twenty-four hours. And from what point is she fit to see [a menstrual flow]? From the time that she grows two [pubic] hairs. Said Rabbi Eliezer, it so happened with a certain girl in Hitlo (Nidd. 9b:8, cf. Yeb. 59b:7), that when her time came to see [a menstrual flow] she skipped over three cycles [before once again resuming her flow], and the matter came before the Sages, and they said, her time is sufficient. They said to him, that was a provisional edict [issued under exigent circumstances, and thus cannot be applied to other case].