Parashas Metzora · Passuk 15:32
Chabad.org — English translation + Rashi
Torah Temima points to Toras Kohanim.
English: “This is the Torah of the zav and one from whom shichvas zera goes out” — the Torah of shichvas zera is equated to the Torah of the zav: just as a zav [has a structure involving] three days, so shichvas zera conveys impurity for three days.
Duration of seed impurity
Chazal link the viable duration of seed impurity to a three-day framework (as discussed in Shabbos regarding the number of onos).