The source's Hanafi reasoning proceeds in steps:
1. ثَلَاثَةَ fixes the count at 3.
2. قُرُوءٍ can refer to menstrual periods1 or periods of purity2.
3. The source selects menstrual periods so that 3 complete counted periods can follow the divorce, without reducing or increasing the fixed number.
4. The limited Usul conclusion is: selecting the evidence-supported meaning of one word must not cancel the exact meaning fixed by another specific word.
This is the source's legal example. The learner is following its reasoning, not deriving an independent divorce ruling.
1. Menstrual period: the days of menstruation.
2. Period of purity: the interval free from menstruation.