Presented by Rav Gidon Ben Moshe of Jerusalem
If a person forgot to include "Reseh" when he recited Birkat Ha’mazon after one of the first two Shabbat meals, he must repeat Birkat Ha’mazon. This Halacha is based on the rule that when a meal with bread is obligatory, one does not fulfill Birkat Ha’mazon after that meal if he omitted the required addition. Therefore, since the Shabbat meals are obligatory, one who forgot to add "Reseh" in Birkat Ha’mazon after one of the Shabbat meals is required to repeat Birkat Ha’mazon.
As mentioned, this applies to the first two Shabbat meals. When it comes to Se’uda Shelishit, different views exist as to whether bread is required. The Shulhan Aruch brings down several different opinions (in Orah Haim 291:5), the first of which requires eating bread, while the second maintains it suffices to eat "Mezonot" food." A third opinion allows eating even foods like meat, and others allow even fruit. According to the opinions that do not require eating bread at Se’uda Shelishit, one does not need to repeat Birkat Ha’mazon if he forgot to include "Reseh" in Birkat Ha’mazon after this meal. Therefore, given the uncertainty involved, a person in this case does not repeat Birkat Ha’mazon.
This Halacha has led to the misconception that as a matter of practical Halacha, one is not required to eat bread at Se’uda Shelishit. But this is incorrect. After the Shulhan Aruch brings the different views mentioned above, he concludes, "Sebara Rishona Ikar" – Halacha follows the first opinion, that one is required to eat bread at Se’uda Shelishit. He adds that if one feels full, and cannot eat bread, he can rely on the lenient opinions and eat other foods. However, the Shulhan Aruch explicitly writes that under normal circumstances, one is required to eat bread for Se’uda Shelishit.
It sometimes happens that people decide they’re too full to eat bread for Se’uda Shelishit, and so they eat other foods, but in truth this is just an excuse, to absolve themselves from reciting Birkat Ha’mazon. (After all, if one has room in his stomach for other food, he has room for bread…) Se’uda Shelishit is a very important Misva, and, as the Shulhan Aruch clearly writes, it should include bread – notwithstanding the fact that there were some Poskim who disagreed, for which reason one who forgets to recite "Reseh" in Birkat Ha’mazon after Se’uda Shelishit does not repeat Birkat Ha’mazon.
Summary: Different views exist as to whether bread is required at Se’uda Shelishit, but the Shulhan Aruch clearly writes that one must eat bread at Se’uda Shelishit, unless one is too full to eat bread, in which case he may fulfill the obligation with other foods.
