Megillah 22B

מיפק לא נפקי.

but they would not leave early, out of deference to Rav, and therefore it was not necessary for him to recite a blessing after he finished his portion. In any event, the incident with Rav does not provide conclusive proof as to the number of readers on a public fast day.

תא שמע, זה הכלל: כל שיש בו ביטול מלאכה לעם, כגון תענית צבור ותשעה באב ⁠— קורין שלשה,

The Talmud tries to adduce another proof: Come and hear the following baraita: This is the general principle: Any day on which labor is permitted and prolonging the prayer service would constitute a deprivation of labor for the masses, For example: a public fast day and the 9th of Av, only 3 people read from the Torah, so as not to lengthen the prayer service unnecessarily.

ושאין בו ביטול מלאכה לעם, כגון ראשי חדשים וחולו של מועד ⁠— קורין ארבעה. שמע מינה.

But any day on which prolonging the prayer service would not constitute a deprivation of labor for the masses, For example: the days of Rosh Chodesh, when it is customary for women to refrain from work, and on the intermediate days of a Festival, when one may not perform labor unless refraining from labor will cause him to lose money, 4 people read from the Torah. The Talmud concludes: Indeed, learn from here that on a public fast day 3 people read from the Torah.

אמר רב אשי, והא אנן לא תנן הכי: זה הכלל, כל יום שיש בו מוסף ואינו יום טוב ⁠— קורין ארבעה. לאתויי מאי? לאו לאתויי תענית ציבור ותשעה באב!

Rav Ashi said: Didn’t we learn in the Mishnah as follows: This is the principle: Any day on which there is an additional offering sacrificed in the Temple and it is not a Festival, 4 people read from the Torah? What is added by the formulation of this principle? Does it not come to add a public fast and the 9th of Av, when there is an addition to the prayer service, and therefore 4 people read from the Torah?