What are the implications of "Yamim Rabim"?
Rashi: It implies three consecutive days 1 - "Yamim" - two days; "Rabim" - a third day.
Irrespective of how many sightings the Zavah has (as opposed to a man, who becomes a Zav after three sightings, irrespective of the number of days).
What does the Torah mean when it writes "be'Lo Es Nidasah"?
Rashi and Rashbam (citing Nidah, 73a): It means that the three sightings occur after the termination of the seven day period of Nidus. 1
Rashi: During the 11 days between one Nidus and the next (i.e. after the end of seven days of Nidah. If she sees after day 11, it begins the next Nidah). See Ba'al ha'Turim (end of Pasuk 26).
QUESTIONS ON RASHI
Rashi writes that she must see three consecutive days to become a Zavah. What is the law if she sees one or two days?
Moshav Zekenim: After one or two days of seeing blood, she guards the next day (refrains from Bi'ah). If she does not see on the guarded day, she is permitted at nightfall, if she immersed. The earliest she may immerse is the morning of the day she guards.