﴿وَالْمُطَلَّقَاتُ يَتَرَبَّصْنَ بِأَنْفُسِهِنَّ ثَلَاثَةَ قُرُوءٍ﴾
Divorced women shall wait concerning themselves for three quruʾ. (Al-Baqarah 2:228)
The textbook uses this verse to illustrate khass. Thalathah (“three”) denotes the determinate number three—not two, four, or an unspecified quantity.
The central point here is the definiteness of the number. The precise interpretation of quruʾ and its legal implications form another question.