Allah تعالى states regarding a divorced woman who menstruates: “Divorced women shall wait by themselves for three quru’.” This verse is the foundation for the ruling of ʿiddah after divorce for women who have regular menstrual cycles.

The key point of difference among the scholars revolves around the meaning of the word quru’. The majority of scholars, including the Shāfiʿī and Ḥanbalī schools, interpret quru’ to mean menstrual periods themselves. According to this understanding, the ʿiddah consists of three complete menstrual cycles.

Based on this view, if the divorce occurred while you were not menstruating, the counting of the ʿiddah begins with the onset of the first menstrual period after the divorce. The first menstrual period counts as the first qur’. The second menstrual period counts as the second qur’, and the third menstrual period counts as the third qur’. The ʿiddah does not end at the beginning of the third menstrual period, but rather at the completion of that third menstrual period, meaning when you become pure from it.

This understanding is supported by the practical application of the Companions and the explanation of classical jurists, who clarified that the purpose of the ʿiddah is fully achieved only after three complete cycles have passed.

By contrast, the Ḥanafī school interprets quru’ to mean periods of purity between menstruations. According to that view, the ʿiddah would end at the start of the third menstrual period. However, since you asked specifically whether it ends at the start or completion of the third menstrual period, it is important to note that the majority position—and the one followed by many contemporary scholars—is that completion of the third menstrual period is required.

Therefore, in your case, counting from the start of the first menstrual period after the divorce, the ʿiddah ends when the third menstrual period has fully finished, and you have entered purity afterward.

Summary ruling:
According to the majority of scholars, if the divorce occurred while you were not menstruating, the ʿiddah ends only after completing three full menstrual periods, meaning at the end of the third menstrual period, not at its beginning.