Since the 9th of Av is on Shabbat, we will fast on Sunday, 10 Av (26 July).
On Shabbat, one may eat meat and wine, until sunset - שְׁקִיעַת הַחַמָּה.
After sunset, one may not eat nor drink anything, until after the fast (Sunday evening).
However, one does not remove one's shoes nor sit on the ground (or low stools) until after the Chazzan has said בָּרְכוּ.
If Maariv begins after Motzai Shabbat time, or one is not going to shul, then one removes one's shoes at Motzai Shabbat time, after saying בָּרוּךְ הַמַּבְדִּיל בֵּין קֹדֶשׁ לְחֹל.
If one eats the 3rd meal after מִנְחָה, then it should not be with a big crowd. One's regular guests are allowed to join, and one may make בִּרְכַּת הַמָּזוֹן with a זִמּוּן.
Source: Kitzur Shulchan Aruch 125:1,2
Sunday, 3 Av 5776