What should one eat for Shalosh Seudos-The menu requirements for the third meal:
The source behind the Shabbos meals: [Although the eating of the Shabbos meals is Rabbinical in origin], it is hinted to from the words in Scripture discussing the falling of the Mun. It is learned from the following verse: “And Moshe told the Jewish people, eat it today, because today is Shabbos for God, as today you will not find it in the field.” The word today, Hayom, is written a total of three times in the verse, and from this repetition the sages derive that there is an obligation to eat three meals on Shabbos.
The obligation to eat bread by the meals: Now, how is this meal fulfilled? Through the eating of bread, [otherwise known today as Lechem Mishneh, even though that in truth having two whole breads is indeed a separate obligation]. This is learned from the fact that the entire source of the three meals is from the verse in Scripture describing the Mun, and the Mun is called bread, hence revealing that also by the three meals one must eat bread. Nonetheless, despite this teaching, we find a difference of opinions regarding the third meal and if indeed eating bread is required.
B. The law:
As stated above, one is required to eat three meals and Shabbos, and hence the meal of Shalosh Seudos is an absolute obligation, and “one must be very careful to fulfill this third meal.” Now, although regarding the first two meals it is a clear ruling, as stated above, that bread must be eaten, regarding the third meal we find a four-way dispute in this matter.
- Bread: Some Poskim rule that one is required to eat bread by the third meal just as is required by the other two meals [which were all derived from the same word of “today” by the verse]. Some Poskim rule that one is required to say Hamotzi on Lechem Mishneh. Other Poskim, however, rule that Lechem Mishneh is not required, and that one can say Hamotzi on a single whole bread.
- Mezonos products: Other Poskim rule that one is not required to eat bread, but one is required to eat a Mezonos product.
- Meat and fish: Other Poskim rule that one is not even required to eat a Mezonos product, and can suffice with eating meat and chicken and fish.
- Fruits and vegetables: Other Poskim rule that one is not even required to eat meat and chicken and fish, and can suffice with the eating of fruits and vegetables.
- A Torah lesson: It states regarding Rabbi Shimon Bar Yochaiy, that when Erev Pesach fell on Shabbos he would spend his time learning the inner dimensions of Torah during the third meal instead of eating any food.
The final ruling in the above debate and the Chabad custom: The final ruling in the above debate regarding the third meal menu, is that one is not to rely at all on the lenient opinions unless it is a time of great need, and is thus to always make sure to eat bread. Furthermore, one is to be careful to eat fish by each one of the three meals, and especially by the third meal.
The Chabad custom: Practically, the custom of the Chabad Rabbeim was to seldomly wash on bread for the third meal and they would rather suffice with simply tasting a random food. This is because the eating of bread for them at this time would serve as a source of distress and contradict the revelation of the world to come that they experienced at that time, and therefore they were not allowed to eat bread which would contradict their Oneg Shabbos. This custom of the Chabad Rabbeim to not eat bread by the third meal is likewise followed by Chabad Chassidim. Nonetheless, while we are not particular to eat bread for Shalosh Seudos, nonetheless, we are particular to eat some other food.
Not all opinions hold that you are obligated to eat bread by the third meal, and the Chabad Rabbeim were even initially accustomed to follow the lenient opinions due to the contradictory feeling that it would create with their Godly experience, if they were to eat bread by the time of the third meal. Those who experience the sublime revelation of the third meal are exempt and even prohibited from eating bread, and likewise the students of such individuals are likewise exempt if they feel distressed over the fact that they can’t follow their teachers’ customs. Even according to the Chabad custom, one must at the very least eat some food for the third meal, even mere fruits.
 Admur 291:8;
 See Admur 274:1 that the scriptural source it is a mere Remez; Kuntrus Achron 271:4; M”A 254:23; However, see Taz 678:2
 Admur 274:1; Rambam Shabbos 30:9; Shabbos 117b; See also Admur 242:4-5; 249:6, 10; 254:6-7; 267:3; 288:2; 291:1 and 5; 334:1-11; 335:1; 429:7; 529:3
 Admur 274:1; Levush 291; Rebbe Yochanon Shabbos 117b
 Beshalach 16:25
 Admur 274:1; Shabbos 117b
 See Admur 274:5
 See Admur 274:2
 Admur 275:5; See Shemos 16:15
 See Admur 291:7; Michaber 291:4; Piskei Dinim Tzemach Tzedek 357; Likkutei Sichos Vol. 21 Sicha 2
 Admur 291:1; Michaber 291:1; Bar Kapara in Shabbos 118a
 Admur ibid; Michaber ibid
The reason for leniency by the third meal: As the third meal is learned from the third time that it states the word today in the verse. Now, in this third time that it states the word today, the verse states “and today you shall not find” emphasizing the negative, that the Mun will not be found on Shabbos. This is in contrast to the first two times that the verse records the word “today” which is in reference to the eating of the Mun. Thus, it makes sense that by the first two meals eating bread is required, as their source in the verse is discussing eating the Mun which was called bread, while by the third meal it is not required being that its source is discussing not finding the Mun bread. [Levush 291:5; See Piskei Dinim Tzemach Tzedek 357] Another reason offered in Sifrei Chassidus, based on the Bach [See Bach 291], is that the third meal corresponds to the time of the future redemption during the era of Olam Haba, and just as in Olam Haba we will not have any eating or drinking [Brachos 17a], so too during the third meal we diminish in eating and drinking. In greater detail this is explained as follows: During the time of the third meal there is a revelation of godliness that comes from the level of “Ayin” which emphasizes the nullification of physicality. Accordingly, it is only befitting to diminish in physical pleasures during this time, and therefore we do not eat bread. It is for this reason that in Scripture we find that the word “today” which refers to the third meal is written in the way of “today it is not found,” to emphasize the point that by the third meal one can fulfill his obligation with the mere taste of food. [See Likkutei Sichos Vol. 21 Sicha 2; Hayom Yom 22nd Adar 1; Meiah Shearim 44a; Keser Shem Tov Hosafos p. 546; Samech Vav p. 545; Ayin Beis 2:1,127]
 1st opinion in Admur 291:7; 1st opinion in Michaber 291:5; Rama 291:4 that so is the custom to do so over bread; Rambam Shabbos 30:9; Hagahos Maimanis; Tashbeitz 22
 1st opinion in Admur 291:7; 1st opinion in Michaber 291:5; Rambam Shabbos 30:9; Hagahos Maimanis Tes and Tashbeitz 22 that so did Mahram
 2nd opinion in Admur 291:7; Shibulei Haleket 93; Tashbeitz 22; Mordechai Remez 397
 The reason: As on the sixth day, two Omrim of Mun per person descended from the heavens and from each Omer they made two loaves, for a total of four loaves for two Omrim. Now, one of the loaves was eating on Friday for the Friday day meal and a second one on Friday night, and a third loaf on Shabbos morning, which leaves us with only one loaf for the third meal. [Admur ibid; Taz 291:4; Mordechai ibid]
 2nd opinion in Admur 291:7; 2nd opinion in Michaber ibid; Opinion in Tur 291; Hagahos Maimanis; Rosh Sukkah 2:13
 3rd opinion in Admur 291:7; 3rd opinion in Michaber ibid; Tosafus Sukkah 27a
 4th opinion in Admur 291:7; 4th opinion in Michaber ibid; Rabbeinu Yona Brachos 36b; Ran 117b; Shibulei Haleket 93
 Zohar 3 95a, brought in M”A 444:2 in name of Shelah
 The reason: This possibility of switching the third meal for learning Torah can be explained based on the fact that the time of the third meal corresponds to the future era in which there is no eating and drinking. Rashbi, who felt this revelation, was therefore able to fast by the third meal. [Likkutei Sichos ibid]
 Admur 291:7; Michaber 291:5; Rambam Shabbos 30:9; Hagahos Maimanis; Tashbeitz 22
 Admur 242:7; M”A 242 in name of Tikkunei Shabbos
 Siddur Admur “It is proper to eat fish by this meal even more than by the other meals”; See Shaar Hakolel 17:25
 See Likkutei Sichos Vol. 21 Sicha 2
 See Hayom Yom 22nd Adar 1; Likkutei Sichos Vol. 21 Sicha 2
 See Admur 288:2-3
The reason: The mitzvah to have a meal on Shabbos is an offshoot of the command of having pleasure on Shabbos. Accordingly, if it were to be true that for whatever reason having a meal does not give the person pleasure, and on the contrary gives him pain, then he is exempt from having a meal on Shabbos, as its intrinsic purpose will not be fulfilled through the eating. Thus, for example, someone who has no appetite and is suffering from a stomach flu and anything he eats gives him tremendous abdominal pain, then not only is he not required to eat the Shabbos meals, but it is practically forbidden for him to do so, in order so he doesn’t cause himself pain on Shabbos. [Admur ibid] This concept does not just apply for the general meal, but also applies to each individual course and food eaten within the meal, and therefore if a certain food does not give one pleasure but rather pain then he is exempt from eating it, and furthermore is even initially prohibited from eating it. Thus, if the eating of bread during the Shabbos meal causes one pain and not pleasure, then he is exempt from eating it, and is not initially allowed to eat it, as doing so would contradict the purpose of the Shabbos meal. [Likkutei Sichos ibid]
 The reason: Although the Chassidim do not associate any pain in eating bread with the revelation of the future, which they do not feel, nonetheless, if they were to eat bread there would be another stress involved, which is mainly the stress of not being able to follow the path of their teacher. Since Chassidim are connected to their Rebbes they deeply desire to follow their customs, and if they would eat bread during the third meal unlike their Rabbeim, this itself would cause them to be distressed and therefore they too are exempt from the eating of bread due to the distress it would cause for them to not be able to follow their teacher’s path. [Likkutei Sichos Vol. 21 Sicha 2]
 The reason: Although in the future era there will be no eating or drinking, and the revelation during the third meal is similar to that which will occur in the future era, nonetheless, until we reach the future era, some level of eating and drinking must be emphasized. The reason for this is because the time the third meal is merely similar to the future era, and does not contain its full revelation, and hence some eating must take place, and this small amount of eating does not contradict the divine experience. Furthermore, in truth, even in the future there will be some level of eating, as is well known that in the future there will be the famous meal of the Levyason and Shur Habur, and hence the concept of eating is not completely negated in the future times, but simply diminished. Furthermore, in truth, in the future era not only will the physical body not be diminished and become secondary, but on the contrary will become elevated and become a source for spiritual fulfillment, for which reason all the souls will come back into their bodies in the times of the resurrection. This is a deeper reason for why we are careful to eat at least something by the third meal, in order to emphasize the greatness of the physical body that will finally be revealed in the future era. [Likkutei Sichos Vol. 21 Sicha 2]