Gemara

(Mishnah): If at the time of Shechitah of a Korban Todah, the bread (40 loaves that must be brought with it) had not yet formed a crust in the oven, the bread is not Mekudash.

Sukah 27a (Mishnah - R. Eliezer): One must eat 14 meals in the Sukah, one each day and one each night;

Chachamim say, only the first night is obligatory.

R. Eliezer says, if one did not eat in a Sukah on the first night, he compensates on the night of Shemini Atzeres.

(Bira, and a Beraisa): He compensates through Targima.

Yoma 79b (Rava): One may eat Peros outside a Sukah.

Support (Beraisa): If one completed (the meal he omitted)) through Targima, he was Yotzei.

i. If Peros must be eaten in a Sukah, the Beraisa should teach that (even) Peros complete his omission!

Rejection #1: Targima are Peros.

Rejection #2: The Tana discusses a place where Peros are not common.

Berachos 49b (Rav Amram): Birkas ha'Mazon is not an obligation on Rosh Chodesh, i.e. one need not eat bread, therefore if he forgot to mention Rosh Chodesh in Birkas ha'Mazon, he need not repeat it.

Question (Rav Avin): According to this, on Shabbos and Yom Tov, when one must eat bread, he should have to repeat it!

Answer (Rav Amram): Yes!

(R. Shila citing Rav): If one forgot (to mention Shabbos or Yom Tov in Birkas ha'Mazon), he must repeat it.

Rishonim

Tosfos and Rosh (Pesachim 101b DH Te'imu and 10:5): Kidush is only where one eats his Seudah. Even if you will say that Targima suffices for Seudah Shelishis, it does not suffice for the primary honor of Shabbos is in the evening and morning meals.

Tosfos (49b DH Iy): Even though one may not fast on Rosh Chodesh, one need not eat bread that obligates Birkas ha'Mazon. On Yom Tov, one must eat bread. In Sukah (27a), we say that one need not eat bread (after the first night)! This means that one need not eat in the Sukah, but he must eat due to Yom Tov.

Question: Since one must eat (bread) due to Yom Tov, and one must eat bread in the Sukah, what difference does it make whether the Chiyuv is due to Sukah or due to Yom Tov?

Answer (Tosfos): Rabbeinu Yehudah says that if it was raining and one ate in the house, he was Yotzei eating on Yom Tov. If the rain stopped, he needs to eat in the Sukah only on the first night, when there is a Chiyuv to eat bread in the Sukah. The Ri was unsure whether one is Yotzei Seudah Shelishis with Targima, like R. Tam explained, like R. Eliezer holds that it helps to complete meals that one must eat the Sukah. Regarding the food one must supply to his wife (if he is away), it seems that all three meals of Shabbos are the same (Kesuvos 64b). Also, since we learn all three of them from the manna, they are the same. However, if one ate a fourth meal and forgot to mention Shabbos in Birkas ha'Mazon, he need not repeat it. Surely, there is no Chiyuv to eat a fourth meal!

Ran (Shabbos 44a DH v'Ika): Some say that one may complete the three meals with Peros and Targima, just like R. Eliezer holds that one may complete with Targima. Targima are Peros. Even though Rava (Yoma 79b) argues and says that Peros do not obligate eating in a Sukah, the conclusion is unlike Rava.

Rebuttal (Rosh Sukah 2:23): Tosfos says that we cannot learn from Sukah to Seudah Shelishis, for the three Shabbos meals are learned from the manna, which was in place of bread.

Poskim

Shulchan Aruch (OC 167:1): Before ha'Motzi, one cuts the loaf a little, so that if he holds up the piece, the loaf will come with it. If not, it is considered a piece.

Mishnah Berurah (6): I.e. if one cuts so much that if he holds up the piece, the loaf will break off, the piece is considered already cut off, even if he could holds up the loaf and the piece would come with it.

Shulchan Aruch (ibid.): On Shabbos, he should not cut until after the Berachah, so the loaves will be whole.

Mishnah Berurah (10): On Shabbos, and also Yom Tov, one must bless on Lechem Mishneh, and we are concerned lest it break before the Berachah, or he will cut so much that the loaf would not come with the piece. Even though not cutting beforehand causes a delay between the Berachah and eating, we are not concerned, for it is necessary. During the week we are stringent to do in the best way (to minimize the delay).

Shulchan Aruch (ibid.): If he cut like he does during the week, it is not a problem.

Mishnah Berurah (11): I.e. if he holds the piece and the loaf comes with it, it is like a whole loaf.

Shulchan Aruch (168:7): Some say that Pas ha'Ba b'Kisnin is made like pockets, filled with honey, sugar, nuts or spices. Some say that it is a dough in which were mixed honey, oil, milk or spices and it was baked, and the mixture of fruit juice or spices is recognized in the dough. Some say that this is proper bread, unless there is so much spices or honey that they are primary.

Taz (7): If there is much honey or milk and only a little water, all agree that this is Kisnin. If there is only a little butter or lard, even if it is tasted, this is proper bread. Therefore, it is fine to use Shmaltz Kuchen (lard cakes) for Lechem Mishneh, since there is only a little lard.

Shulchan Aruch (274:1): (On Shabbos night) one blesses on two whole loaves.

Mishnah Berurah (1): The two loaves commemorate the manna. Also on Yom Tov one must bless on two loaves. This is obligatory also for women, since the miracle of manna was also for them.

Mishnah Berurah (2): If a loaf was slightly burned, and nothing was cut off, some say that one is Yotzei with it for Lechem Mishneh. If one does not have a whole loaf, this is not Me'akev. One may bless even on a k'Zayis of bread.

Kaf ha'Chayim (4): If a loaf is missing the Shi'ur of Chalah (one part in 48), opinions differ about whether or not this is called whole. If one does not have other whole loaves, it is better to use it for Lechem Mishneh than to bless on only one loaf. If one has two half-loaves, he should join them to make them look whole.

Kaf ha'Chayim (5): If mice nibbled on a loaf, one is not Yotzei (Lechem Mishneh) with it, for it is clear that what was taken was not for the sake of Chalah. Also, it is repulsive. Even if one removes more, so it is not recognized that mice nibbled on it, this does not help, for it came from a Pasul loaf.

Kaf ha'Chayim (6): If a loaf was kneaded with lard, even if it is the majority and it is Pas ha'Ba b'Kisnin, this is called bread, for if one fixes a meal on it he blesses ha'Motzi. The Taz says that one is Yotzei with it, i.e. if it is merely for Lechem Mishneh, but he will not eat from it. Stam it is not called bread, for people do not usually fix a meal on it. When one will eat from it, one is Yotzei with it. If one has no other loaf, it is better to use it for Lechem Mishneh (even if he will not eat from it) than to bless on only one loaf.

Kaf ha'Chayim (7): If it did not form a crust in the oven, it is not called bread. Such Lachmei Todah do not become Kodesh. We learn from here that one is not Yotzei with it for Lechem Mishneh.

Kaf ha'Chayim (8): Beis Meir says that one is Yotzei even with a loaf less than a k'Zayis. Kaf ha'Chayim (Palagi, 36) and Pesach ha'Dvir disagree.

Kaf ha'Chayim (9): If two loaves stuck together, and one was cut and part of it is stuck to the other, which is whole, the stuck part should be cut off so the loaf will look whole.

Shulchan Aruch (4): The night and morning Seudos must be with bread.

Beis Yosef (DH v'Seudah): Semag requires three meals of bread, since the verse we learn from (Shabbos 117b) mentions bread.

Hagahos (in Tur ha'Shakem, 4): He means that the verse discusses manna, which is called bread (Shmos 16:15).

Mishnah Berurah (9): Even the lenient opinion (291:5, below) regarding Seudah Shelishis agrees that the first two meals, which are the primary honor of Shabbos, must be with bread. If due to Ones one cannot eat at night, the meal is delayed until the day. He eats three meals during the day. However, he must say Kidush at night, and immediately after Kidush eat a k'Zayis of the five grains or drink a Revi'is of wine.

Kaf ha'Chayim (24): Some say that even at the first two meals, one is Yotzei with Pas ha'Ba b'Kisnin even if he eats less than Pras (half a standard loaf of six or eight eggs) and blesses Borei Minei Mezonos and Al ha'Michyah on it. Yosef Ometz (48) disagrees.

Shulchan Aruch (291:5): Seudah Shelishis must be with bread.

Kaf ha'Chayim (30): This is even if Shabbos is Erev Yom Tov.

Shulchan Aruch (ibid.): Some say that it can be with any food from the five grains.

Beis Yosef (DH v'Yesh): The Ran says that Targima suffice, and also R. Yonah (Berachos 36b) in the name of Chachamim of France. Tosfos, the Rosh and Hagahos Maimoniyos rejected the proof. The Mordechai brought both opinions and did not decide the matter. Semag says that all three meals require bread.

Magen Avraham (10): The five grains suffice only if one blesses on them Borei Minei Mezonos.

Gra (DH v'Yesh): Rashi says that Targima are Peros, like the first answer (rejection) in Yoma. Tosfos argues, for the Halachah follows Rava. The Ran says that the Halachah does not follow Rava, rather, like the Gemara's rejection in Yoma. The Rosh says that Targima are Mezonos, like a Tosefta which says that one blesses on Targima Borei Minei Mezonos.

Mishnah Berurah (23): All agree that the ideal Mitzvah is with bread, and with two loaves. They argue about b'Di'eved, if one is Yotzei without bread.

Shulchan Aruch (ibid.): Some say that it can be with foods used to accompany bread, e.g. meat and fish, but not Peros. Some say that it can be even with Peros. The first opinion is primary, that it must be with bread, unless one is too satiated.

Mishnah Berurah (24): The latter opinions are lenient because Seudah Shelishis is not normally eaten for satiation.

Kaf ha'Chayim (33): Rabbeinu Yehudah says that it must be with Peros of the seven species.

Rema: Also, (it need not be with bread) when one cannot eat bread, e.g. Erev Pesach that is on Shabbos, for one may not eat bread after Minchah.

Mishnah Berurah (25): Therefore, one fulfills Seudah Shelishis with foods made from fried Matzah crumbs, or with meat, fish or Peros.