The 3 Fasts Min Hatora In Teves

The  Shulchan Aruch (580:1) enumerates  21 additional fast days (on top the 4 fasts, Ta'anis Esther  & Yom Kippur)

Two of them are:

On the 8^th day in Teves.  The Torah was written in Greek.

On the 9^th day in Teves. Reason for it is unknown.*

The Tur quotes from the Behag  “These fasts are Min Hatorah.

The Bach explains, “since some of these fasts fall on Chanukah and Rosh Chodesh, therefore they are all considered  like Yom Kippur which is Madche Shabbos and is D’orysa.”

The Bach gives a second explanation. “There is a Mitzvah Min Hatorah to blow Shofar Daven and fast, Al Kol Tzoroh Shelo Tavoh.

The Mitzvah of fasting is even after the Tzoro went away”.

According to both explanations, Asoro B’Teves would qualify to be called a D’orysa.**

Hence we have the 8^th ,9^th and the 10^th of Teves, 3 days of fasting Min Hatorah,  

* Plan to write on this subject

** According to the 1st expl there are four

fasts D'rabanan 1) T Esther  2)Tzom G.

3)17 Tamuz    4) 9 Av

According to 2nd expl. there is only one

fast D'Rabanan T.Esther (אמבצי)