The earlier lesson treated khass as wording assigned to a determinate meaning, illustrated by thalathah, the number three. Mushtarak raises an extra question because the word has several distinct lexical assignments.
Thalathah identifies the number three.
Qurūʾ names periods whose relevant sense may be menstruation or purity.
Thus in thalāthata qurūʾ, understanding the number and identifying the type of periods counted are different interpretive tasks.