Two questions:
1. Rabbeinu Tam's opinion of 72 (or even 96 minutes) for night after the
setting of the sun contradicts known facts. Obviously half an hour
after sunset in the springtime in Jerusalem the entire sky is quite dark
with numerous stars. It is well known that the "bein hashamosot" is much
shorter in Jerusalem (or babylonia) than in northern Europe.
The Gra in Shulcahn Arukh notes that the time between dawn and sunrise
differs much from the time betwwen between sunset and stars. However,
he doesn't seem to be bothered by the little time it takes 3 stars
even small stars to appear.
2. The Gemara later ends up with "teuvta" on Rav. Why doesn't the Gemara
give the usual answer that Rav is also a Taana and can disagree with
a Beraisa?
Kol Tuv,
Eli Turkel
1.The Vilna Ga'on (Shenos Eliyahu, beginning of Berachos) indeed asks explicitly that it does not take that long for three stars to appear, and he answers, like you wrote, that it depends on the geographical location.
2. The YAD MALACHI (#544) writes (from the Acharonim, and from the Ritva at
the beginning of Maseches Eruvin, and from the Shitah Mekubetzes in Bava
Metzia) that the Gemara only gives the answer of "Tana Hu u'Palig" when
there is no other answer at all. It is considered to be a "forced" answer.
It could be that the Gemara could have answered here that Rav disagrees with the Beraisa, but since the Halachah is not like Rav in this case, the Gemara did not strain itself to give this forced answer.
-Mordecai