Purity: Continuity of the First Three Days of Ḥayḍ
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 439:
The first three days of ḥayḍ must be continuous; therefore, if, for example, a woman experiences bleeding for two days, and then the bleeding stops for one day, and then she experiences bleeding again for one day, it is not ḥayḍ.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)