Question:How do we reconcile the statement of Rav Yosi, who says that Bain Hashmoshis lasts only like a "blink of an eye" (and, as Tosofos explains on 34b, that he agrees we don't know when this occurs),with the statement of Rav Yehudah in the name of Shmuel, that night comes with the appearance of three stars (particularly according to the second side of Rav Elchonon Wasserman's z'tl "chakirah" that three stars MAKE night)? Or, are these two statements mutually exclusive?
There is no contradiction there. Bein ha'Shemashos does not begin when the first star appears and end when three stars appear. Rather, it begins and ends when three stars appear.
However, from the Gemara (35b) that says that Bein ha'Shemashos of Rebbi Yosi begins after the conclusion of the Bein ha'Shemashos of Rebbi Yehudah, it implies that it is indeed after the appearance of the third star.