Gemara

(Beraisa - R. Eliezer ben Yehudah citing R. Yehoshua): When Erev Pesach falls on Shabbos, we destroy everything before Shabbos, including Terumah. We just leave enough Tahor Terumah for two meals that will be eaten before the fourth hour.

Shabbos 118a (Mishnah): If there was a fire on Shabbos afternoon, one may save one meal.

Suggestion: It was before eating.

Rejection: No, it was after eating.

(Beraisa): One may wash bowls that he used Shabbos morning for the sake of Tzaharayim (noon). One may wash bowls used b'Tzaharayim for the sake of Minchah time. One may not wash bowls used at Minchah.

Rishonim

Rambam (Hilchos Shabbos 30:9): One must eat three Shabbos meals, one at night, one in the morning and one at the time of Minchah.

Rosh (16:5): One may wash bowls that he used Shabbos morning for the sake of (Bach adds 'Tzaharayim. One may wash bowls used b'Tzaharayim for the sake of) Minchah time. One may not wash bowls used at Minchah [since he will not eat another meal on Shabbos]. This shows that Seudah Shlishis is not before Minchah. Also Pesachim 13a supports this. It seems that when Erev Pesach is on Shabbos, we leave Chametz for two meals. We need not leave for Seudah Shlishis, for Chametz is forbidden after six hours. This is no proof. Even if one divides the morning meal into two (to fulfill also Seudah Shlishis), he eats only the amount that he normally eats in one meal!

Ran (43b DH Kamah): Bahag says that the meals need not be in the morning and afternoon. Even if he ceased his meal and said Birkas ha'Mazon, even if he did not remove the tray (of food), just spread a cloth and blessed, says ha'Motzi and eats k'Beitzah (the volume of an egg) and blesses Birkas ha'Mazon again, this is fine. The Gemara permits washing dishes used b'Tzaharayim to use at the time of Minchah. This is merely a typical case [but if one ate all three meals already, he may not wash them].

Poskim

Shulchan Aruch (OC 291:2): The time for Seudah Shlishis is from six and a half hours. If one ate before this, he was not Yotzei.

Beis Yosef (DH u'Zmanah): The Ran says that one may interrupt his meal, bless Birkas ha'Mazon, and then bless ha'Motzi again, in order to fulfill three meals. Tosfos, the Rosh, Mordechai and Hagahos Maimoniyos say that the time is only after Minchah.

Beis Yosef (DH v'Chen): The Rosh (Teshuvah 22:4) says that he saw his Rebbeyim split the morning meal [when necessary], and so he did. Even though Tosfos brings from the Ri not to do so, due to the Isur of Berachah she'Einah Tzerichah, since he does so to honor Shabbos [through fulfilling three meals], this is for a need. The Rosh also wrote (22:13) not to publicize this, because before midday, one may not split the meal, and the Tzibur are not so exact about this. In winter, Tashbatz (Katan 21) would eat Seudah Shlishis right after Birkas ha'Mazon.

Magen Avraham (3): If one began before midday and ate until after midday, he was Yotzei, for he ate after midday. However, if one knows that the meal will last after midday, it is forbidden, like I wrote in 471:1. This is unlike Olas Shabbos.

Rebuttal (Eliyahu Rabah 4): Why was he Yotzei? If he started before midday, this is like splitting the meal!

Note: Perhaps he means that he is Yotzei the morning meal, and in order to be Yotzei also Seudah Shelishis, he must say Birkas ha'Mazon in the middle. The Magen Avraham connotes that he is Yotzei both with without doing so.

Eliyahu Rabah: It depends on the beginning, like we say in 188:10. (If one ate during the day, and it was Shabbos [or Rosh Chodesh...], and blesses at night, he mentions Shabbos [or Rosh Chodesh] in Birkas ha'Mazon.) However, if he started after midday and continued eating after six and a half hours, he was Yotzei.

Machatzis ha'Shekel: The Magen Avraham's law is according to the Beis Yosef, who forbids even a small meal before praying Minchah. In 471:1, the Magen Avraham said that if one may not start eating [after nine hours on Erev Pesach], one may not plan to start eating beforehand and the meal will last until then.

Eliyahu Rabah: Why does the Magen Avraham forbid starting if the meal will continue after midday? In 471, we forbid lest he not have appetite at night. This does not apply to Minchah! (If one started b'Heter, he may continue.)

Kaf ha'Chayim (11): He was not Yotzei then, for it is the time for the morning meal.

Kaf ha'Chayim (12): We hold like the opinion that he was not Yotzei.

Gra (DH Zmanah): The Gemara assumed that in the afternoon, one had not yet eaten Seudah Shlishis. It permits washing dishes then for the sake of Seudah Shlishis.

Mishnah Berurah (7): If the meal began before midday, and continued until after the time for Minchah Gedolah (six and a half hours), he was Yotzei, for he ate in the time. It does not depend on when he began.

Mishnah Berurah (10): The first opinion permits a small meal before Minchah.

Kaf ha'Chayim (14): The Ari Zal says that Seudah Shlishis must be after Minchah.

Rema: One may not drink water between Minchah and Ma'ariv on Shabbos, for then Neshamos return to Gehinom. Therefore, one should not eat between Minchah and Ma'ariv. Rather, he eats before Minchah.

Beis Yosef (DH v'Rabbeinu): R. Tam enacted not to eat Seudah Shlishis between Minchah and Ma'ariv.

Kaf ha'Chayim (13): The Mechaber omitted this, and also the Rambam. Even those who forbid agree that if one began, he may eat until night.

Rema (ibid.): Some say that it is better to pray Minchah first. This is the custom l'Chatchilah in all these lands.

Magen Avraham (5): One may not eat before praying. R. Tam permits a small meal.

Mishnah Berurah (11): If it will be hard to fulfill Seudah Shlishis after Minchah, l'Chatchilah one may fulfill it before Minchah, if it is before Minchah Ketanah (nine and a half hours).

Gra (DH v'Yesh): A proof is from Pesachim 105a. Shabbos is not Kove'a for Havdalah. This is only [for other drinks, but not for water. This shows that one may eat after Minchah until night, and even drink water - Damesek Eliezer.]

Rema (ibid.): In any case, one should not drink water from the river. One may drink in the house. All the more so, other liquids are permitted. Some forbid only within 12 months of the death of one's father or mother. Some say that the Isur is only on Erev Shabbos.

Kaf ha'Chayim (15,16): Since this involves danger, one should be concerned for both opinions, and not drink Bein ha'Shemashos of Erev Shabbos and Motza'ei Shabbos. We forbid only water drawn from the river at this time. On Erev Shabbos one may not drink because he must say Kidush first, and on Motza'ei Shabbos it is forbidden due to croup.

Rema (3): One who knows that he can eat Seudah Shlishis after praying with the Tzibur may not eat Seudah Shlishis before Minchah. However, if he did, he was Yotzei.

Magen Avraham (8): Even if Yom Tov is on Motza'ei Shabbos, if one will be able to eat afterwards before the 10th hour, he should not split the morning meal, for some forbid doing so due to an unnecessary Berachah.

Kaf ha'Chayim (21): The Magen Avraham connotes that Seudah Shlishis must be with bread, even when it is Erev Yom Tov (surely, this is the correct text - PF). Sa'if 5 connotes like this. He connotes that it does not depends on before or after Minchah, rather, before or after the 10th hour.

Mishnah Berurah (16): I.e. he estimates that he will be able to eat later without gorging himself.

Mishnah Berurah (17): I.e. if later he will be able to eat more than k'Beitzah, he does not split the meal. If one has just enough for two meals, and if he splits them into three meals, he will not have enough for satiation at each, it is better to eat two meals to satiation. If the morning meal extends after midday, he should split it, for he will be satiated after the latter half. L'Chatchilah he should not delay until after midday, for he is Mevatel Oneg Shabbos.

Bi'ur Halachah (DH Achar): I did not rule like the Magen Avraham [who says that it depends on 10 hours], for one may eat even at the end of the day. Only fixing a meal is forbidden. One may eat bread more than k'Beitzah, even if Yom Tov follows a weekday. It is very difficult to say that merely due to the Mitzvah of Shabbos, it is considered a fixed meal, like we find elsewhere that Shabbos fixes. This does not apply here. Here we want that at night, he will eat with appetite! The Gra (on 529:1) says so.