When Tisha B’Ab is observed on Sunday, as it is this year, when Tisha B’Ab falls on Shabbat and is thus delayed until Sunday, the Se’uda Mafseket (last meal before the fast) is the Se’uda Shelishit meal eaten on Shabbat afternoon. The Bet Yosef (Orah Haim 552) writes that in such a case, one may eat and drink at this meal as much as he likes. He may eat meat and drink wine, and he may even, in the words of the Bet Yosef, partake of a lavish meal "like the meal of King Shelomo in his time." The Mishna Berura (Rabbi Yisrael Meir Kagan of Radin, 1839-1933), citing the Tur, adds that one who wishes to indulge at this meal may do so and should not be prevented from eating as much as he likes. The Mishna Berura also notes that those who customarily eat Shabbat meals with a large group of family or friends, such as a congregation that conducts a communal Se’uda Shelishit every week, may do so even for the Se’uda Shelishit before Tisha B’Ab. Although the Se’uda Mafseket is generally eaten alone, when Tisha B’Ab begins on Mosa’eh Shabbat one may conduct a large, festive meal with other people for Se’uda Shelishit.
However, one should not explicitly state when he eats Se’uda Shelishit that he is eating in order to help him fast. It is forbidden to make preparations on Shabbat for after Shabbat, and thus although one certainly may eat and drink as he wishes on the Shabbat before the fast, he should not verbally state that this is being done in preparation for Tisha B’Ab. This Halacha is mentioned by the Mishna Berura (290:4) and in Shemirat Shabbat Ke’hilchatah (28:77). Likewise, one may not prepare on Shabbat books that he will need on Tisha B’Ab. For example, one should not remove the Kinot books from the shelf so they are ready for Tisha B’Ab, unless he intends to read them on Shabbat. One should also not prepare his non-leather shoes on Shabbat.
Otherwise, the Shabbat in such a case is treated as an ordinary Shabbat, even though it is actually the 9th of Ab.
Summary: When Tisha B’Ab begins on Mosa’eh Shabbat, one may eat and drink as much as he wishes on Shabbat, without any restrictions, even for Se’uda Shelishit, and he may also eat this meal with a large group if this is his usual practice. One should not, however, explicitly state that he eats in preparation for the fast, or make any preparations for the fast, on Shabbat.