1. The Laws Pertaining to the Three [Shabbos] Meals, 6 Seifim: One should be very cautious to have a third meal, and even if he is satisfied he can fulfill it with about the volume of an egg. And if he cannot eat at all, he need not cause himself discomfort. And a wise person will not fill his stomach with the morning meal, so as to save room for a third meal. RAMA: And one who did not eat on Shabbat evening should eat three meals on the day of Shabbat. (the Rosh)
2. The time [of this meal] is from the time of minchah, which is from the sixth and a half hour onward. And if he does this earlier, he has not fulfilled the mitzvah of the third meal. RAMA: There are those who say that it is prohibited to drink water from Minhah until Maariv on Shabbat for that is when the souls return to Gehinom. And therefore one should not eat the third meal between Minhah and Maariv. Instead he should eat it before Minhah (Tosafot; the Rosh and Mordecai, chapter ten of Pesahim). And there are those who say that it is is preferable to pray Minhah first (Rambam, Tur, Hagahot Mordecai, Hagahot Maimoni, chapter 30 and Agur). And this is our preferred custom in all of these lands. And in any case one should not drink water from the rivers, but it is permissible in one's house. And all the more so other liquids are permissible (Hagahot Maimoni, chapter 30). And there are those who say that this is prohibited only during the twelve months after one's mother or father died (Agudah and Mordecai, chapter 10 of Pesahim). And there are those who say that this prohibition of drinking water is only on Friday eve (Tosafot, Rosh, and Mordecai in the name of R. Meshulam, and Hagahot Maimoni.)
3. If one’s morning meal is extended until the time of minchah, one should stop the meal, bless birkat hamazon, wash one's hands, say Hamotzei and then eat (the third meal). If one does not conduct himself in this manner, since the morning meal was extended this late, he will not be able to partake of the third meal except by eating uncomfortably. RAMA: However, one who knows that he is able to eat after saying Minchah with the congregation does not eat the third meal before Minchah; but if he did so, he has fulfilled his obligation.
4. It is not necessary to make kiddush during the third meal, but one must break bread over two loaves. RAMA: Even if one partakes of many meals on Shabbos, it is necessary to have two loaves for every meal (Avudraham; Maimoni). There are, however, authorities who maintain that even for the third meal, it is sufficient to have one complete loaf [Tur; Mordechai]. Accordingly, it has become widespread custom in these countries to rule leniently and to break bread for the third meal on one whole loaf. But there is what to be stringent in, to take two (loaves).
5. One must use bread. And there are those who say that one may use whatever food is made from the five types of grain (i.e. Wheat, barley, rye, oats, and spelt). And there are those who say that one may use things that go with bread, such as mean and fish, but not with fruit. There are some who say that even fruit may be used. The first opinion is the essence, that one must use bread unless he is really full. RAMA: Or in a place where it is impossible for one to eat bread, like on Erev Pesach that falls on Shabbat, where it is forbidden to eat bread after Minchah, as was brought in the laws of Pesach [Mahar"il, Laws of Pesach].
6. Women are obligated in the third meal.