The Torah in Parashat Beshalach tells about the manna, the miraculous food which fell each morning for Benei Yisrael during their years of travel in the wilderness. The first day the manna fell, Moshe warned the people not to leave over any manna for the following day, and those who did found that the leftover manna had spoiled overnight. The exception was on Friday, when double the daily quantity was provided for each person, and the extra portion remained intact until the following day, Shabbat, when no manna fell. The first Shabbat after the manna began falling, when the people discovered that the leftover portion remained fresh, Moshe instructed them, “Eat it today, for today is Shabbat for the Lord; today, you will not find it in the field” (16:25).
The Gemara in Masekhet Shabbat (117b) famously infers from this verse the obligation of shalosh se’udot – to eat three meals on Shabbat. Moshe mentioned the word “hayom” (“today”) three times in his instructions to Benei Yisrael to eat the manna on Shabbat, alluding to a requirement to eat three meals over the course of Shabbat.
Rav Yosef Chaim of Baghdad (the “Ben Ish Chai”), in his Ben Yehoyada commentary, notes that rather than simply state this obligation, the Gemara first poses the question, “How many meals is a person required to eat on Shabbat?” It then answers, based on the aforementioned verse, that one must eat three meals. The Ben Yehoyada finds this presentation significant, and suggests that it indicates that all three meals constitute a single mitzva. Had the Gemara simply stated, “A person is obligated to eat three meals on Shabbat,” we would conclude that a person is obligated three times on Shabbat to conduct a meal, such that each meal fulfills its own, separate requirement. But the Gemara poses the question, “How many meals is a person required to eat on Shabbat,” which the Ben Yehoyada explains to mean, “How many meals is a person required to eat on Shabbat to fulfill his obligation to enjoy Shabbat?” Meaning, according to the Ben Yehoyada, the three Shabbat meals comprise a single obligation, such that if a person eats just one or two meals, he has not fulfilled a mitzva at all. Since there is but a single mitzva to eat three meals, one is not credited with any mitzva if he eats fewer than three meals. Therefore, according to this view, if one knows he will be unable to eat three meals on Shabbat, either because he does not have enough food, or because his medical condition does not allow him to eat three meals, there is no halakhic value in eating one or two meals. Since the three meals are components of a single mitzva, one who eats fewer than three meals does not fulfill any mitzva, and there is thus no value in eating only one or two meals without the third.
This also appears to be the position of Rav Chaim Binyamin Pontremoli, in his Petach Ha-devir (3:274:1), where he addresses the question of why the Sages did not institute the recitation of a berakha over the mitzva of shalosh se’udot. He answers by drawing a comparison to the mitzva of the four cups of wine on the night of the seder on Pesach, over which no berakha is recited. The Avudraham, as the Petach Ha-devir cites, explains that the Sages enacted the recitation of a berakha over a mitzva which we perform without any interruption, and thus no berakha is recited over the mitzva of the four cups, which are drunk at different stages over the course of the seder. By the same token, the Petach Ha-devir writes, since the mitzva of the three Shabbat meals is fulfilled on three separate occasions over the course of Shabbat, no berakha was instituted over this mitzva. This comparison would certainly suggest that, like the Ben Yehoyada, the Petach Ha-devir understood that one does not fulfill a mitzva at all by eating fewer than three meals on Shabbat, as they are three interdependent components of a single requirement.
However, Rav Chaim Chizkiyahu Medini, in his Sedei Chemed (Pe’at Ha-sadeh – Ma’arekhet Berakhot, 3) cites those who disagree, and maintain that each Shabbat meal constitutes a separate, independent obligation. And thus, according to this view, even one who cannot eat the third meal, for whatever reason, should eat two meals, as he thereby fulfills two mitzvot.
(Based on Rav Asher Anshel Schwartz’s Ma’adanei Asher, Parashat Beshalach, 5778)