Zavim, Chapter One, Mishnah Three
If he saw one issue on one day and two on the next day, or two on one day and one on the next day, or three on three [consecutive] days, or three nights, he is a full zav.
This mishnah lays out the bottom line rule with regard to what is required for a person to become a full zav. If he has a three discharges on up to three days or nights straight, no matter what the combo, he is a full zav. He must bring a sacrifice at the end of his period of cleanness.
We should note that this is different from a zavah. For a woman to become a zavah she must have non-menstrual blood flow on three consecutive days. Three times on one day does not make her a zavah.