In Meseches Shabbos, daf kuf yud zayin, amud bais it says that a person may save food for three meals on Shabbos from an active fire. One is permitted to save food for humans as well as for his animals. The gemara clarifies this by saying if the fire takes place at night one may save for three meals. If the fire takes place in the morning, one may save for two meals. If the fire takes place during the time of mincha on Shabbos afternoon, one is permitted to save food for only one meal. The gemara explains that a person needs to eat three meals on Shabbos. Rav Chidka says a person needs to eat four meals on Shabbos. They both learn it from the posuk where Moshe commanded Bnei Yisroel to eat the meals on Shabbos and Moshe mentions the word “Hayom” three times. The Tana Kama says we need to eat three meals. Rav Chidka says the words are separate from the night meal, so there are a total of four meals.
The Rambam in Hilchos Shabbos, Perek Lamed, halacha tes says that one is required to eat three seudos on Shabbos; one in the evening, one in the morning, and one at mincha time. The Magid Mishna explains the Rambam as saying that one must eat the seudos in their proper time and these are the zmanim for doing so. This is a machlokes of the Geonim as the Halachos Gedolos in Perek Tes Zayin paskens that one may split his morning meal, bentch and wash again. He would then be yotze the third meal in the morning. Based on the Halachos Gedolos, the Ramban and Rashba say that the zmanim are not hard zmanim and one may eat three meals, even if they are not in the respective times that the Rambam mentioned.
The gemara in Shabbos, daf kuf yud ches, amud alef says that one may rinse out the plates used on Friday night for use on Shabbos morning. In the same vein, the dishes used for the day meal may be washed in order to be able to be used for the third meal. It is prohibited to wash the dishes after the third meal. A cup may be washed the entire day since there is no set time when it comes to drinking. The Ramban and Rashba say that the gemara is only saying the norm, not that one has to eat during these times.
The Aruch Hashulchan in siman resh peh ches, seif bais writes that the second seuda should be eaten before chatzos the same way the third meal needs to be eaten after the zman of mincha. He wonders why none of the poskim bring this down lehalacha. The Aruch Hashulchan finishes off saying that despite that it is not brought down in the poskim; nevertheless, the second seuda must begin before chatzos. He adds that this is his minhag.
The Bach in siman shin lamed daled says that Rashi in the gemara in Shabbos explains that if a fire took place in the evening before eating the seuda of Shabbos, since the entire night is the appropriate time for eating the night meal, even if the fire took place late at night it would be permissible to save enough for the seuda. When it comes to the morning, Rashi says the lashon of “before the seuda” not the “eating of the seuda,” because the morning meal must be eaten before chatzos. The Maamar Mordechai argues with this position and says that really there is no problem with eating the meal after chatzos, it is just an issue that one is not allowed to fast on Shabbos. A person therefore has to eat before chatzos. In a case of a fire, if one did not eat before chatzos it is permissible to save for both meals. The Pri Megadim and Mishan Berurah seem to agree with this psak, that so long as one ate something before chatzos and the person is not fasting, then the person may eat the morning meal even after chatzos.
It would behoove us all to eat the morning meal in the proper time but if not, then one has on whom to rely as long as the person is not fasting.