There are various positions on when we can say maariv (evening prayers), corresponding to various positions on when the day ends (and, thus, a new day begins which starts from the night). But the times I see for starting the Passover seder all use a stringent (i.e., late) definition (3 stars, even 3 small stars). Why don't we (or when do we) use a lenient definition of nightfall on seder nights?
מיד כששקעה חמה יכול להתחיל בסדר אך לא יאכל המצות קודם צאת הכוכבים
Immediately when the sun sets one can begin the seder, but one should not eat the matzot before tzeit hakochavim.
In a footnote there he explains:
כיוון דביה"ש הוא ספק לילה במצה שהיא דאורייתא צריך להמתין לצאה"כ
Since bein hashemashot is possibly night, by matzah which is biblical you have to wait until tzeit hakochavim.
The logic here seems to be that since it is uncertain whether night already begins at sunset, we apply the rule of acting stringently for biblical mitzvot and acting leniently for rabbinic mitzvot.
The Mishna Brura Orach Chaim Siman 472,1 say that one should wait till definite nightfall, not Shekia-sunset (doubtfull nightfall), which means only when 3 stars emerge at night.
According to Myzmanim there are various opinions when this is, some say 6.45 degrees, some 7.12 degrees according to Igros Moshe or 72 minutes for those that hold like Rabeinu Tam. Even though on Motzei Shabbos and Yom Kippur is there a minhag to use the more strict definition of Nightfall 8.5 degrees because there is a mitzva Deoraisa and punishment of Kareis (unless there is need for someone unwell), on Seder night though, the Hagadda which has to accompany the matza at Nightfall is Deoraisa, but by the time you get to Ma Nishtana after Kaddesh, urchatz, Karpas, Yachatz and pouring the second cup, the 7 or 8 minute difference between 7.12 degrees lenient time to 8.5 degrees strict time is easily covered, so one can use the lenient opinion unless one is of a Chassidus that always keep 72 minutes after sunset.