Question: Is there any obligation or fine custom to recite Kiddush during Seuda Shelishit (the third Shabbat meal) just as we do before partaking of the Shabbat day meal?
Answer: The Rambam (Chapter 30 of Hilchot Shabbat, Halacha 9) states: “One must partake of three meals on Shabbat. One must likewise establish every one of these meals upon wine and break two loaves of bread.”
It would seem that according to the Rambam, just as there is an obligation to recite Kiddush before the first two Shabbat meals, i.e. the Shabbat night and Shabbat day meals, there is likewise an obligation to recite Kiddush before partaking of the third Shabbat meal as well.
The Tur (Chapter 291) quotes the words of the Rambam and notes, as follows: “The Rambam writes that one must establish the third Shabbat meal upon wine. However, my master and father, the Rosh, would not recite a blessing on wine before Seuda Shelishit, for Kiddush by day is compared to Kiddush at night: Just as one Kiddush on wine is sufficient for the entire night, so too, reciting Kiddush on wine during the day is sufficient for the entire day.” This means that the Tur also understood that the Rambam requires Kiddush to be recited before Seuda Shelishit; however, he disagrees and writes that reciting Kiddush on wine on Shabbat morning is sufficient for the entire day and one need not recite Kiddush again before Seuda Shelishit.
Indeed, several other Rishonim understood the words of the Rambam to mean that one must recite Kiddush on wine before Seuda Shelishit. Nevertheless, Maran Ha’Bet Yosef writes that the Tur’s understanding of the Rambam that he requires Kiddush to be recited before Seuda Shelishit is incorrect, for this is not what the Rambam meant. Rather, the Rambam only meant that one must drink wine during Seuda Shelishit but not that one must recite Kiddush at the beginning of this meal. The Bet Yosef proceeds to support his view based on the words of the Rambam in a different Halacha.
Similarly the Radbaz (Volume 5, Chapter 2104), who lived in the same generation as Maran Ha’Bet Yosef, writes that it is clear that the Rambam agrees that there is no obligation to recite Kiddush before Seuda Shelishit. Other great Poskim write likewise.
Maran Rabbeinu Ovadia Yosef zt”l deals with this issue as well (in his Chazon Ovadia-Shabbat, Part 2, page 341) and concludes that there is no obligation to recite Kiddush on wine before Seuda Shelishit in accordance with the prevalent custom of the Jewish nation which is to recite Kiddush only before the first two Shabbat meals. There is no obligation to recite Kiddush before Seuda Shelishit and it is merely a pious custom to drink some wine during Seuda Shelishit in order to fulfill the Rambam’s opinion who maintains that this is obligatory at least some time during the meal.
Maran zt”l adds that although some claim that according to the Kabbalah, one should recite Kiddush before Seuda Shelishit, this is incorrect; he proceeds to prove from the words of the saintly Ari z”l that Kiddush should not be recited before the third Shabbat meal. It is, nevertheless, proper to drink some wine during Seuda Shelishit according to the Kabbalah. This fits perfectly with the aforementioned opinion of the Rambam.
Summary: One should not recite Kiddush before partaking of Seuda Shelishit; however, it is preferable that one drink some wine during this meal.