Q. If we always eat shaleshudes in shul should we also do it on this Shabbos when it is a nidche or is it better to eat at home?

A. See prior question. Poskim disagree if it is better to eat with company on the Shalosh Seudos meal on Tishaa Beav – nidche. Mogen Avrohom (552: 14) and Aruch Hashulchan (ibid. 12.) opine that it is better to avoid being with company. Other Poskim (Bechor Shor – Taanis 30Shaarei Teshuva 552: 11) maintain that if you eat every Shabbos this seuda in shul, omitting now may be seen as Avelus Befarhesia (explicit mourning that is prohibited on Shabbos.)
Horav Shlomo Miller’s Shlit”a opinion is that it is preferable to eat the Third Seuda at home.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a