Guest post by Daniel
Shulchan Aruch 291:1
Din Shalosh Seudos
A person should be very cautious to fulfill [the mitzvah of eating] the Third Meal [on Shabbos].
As is brought in the gemara: A person is obligated to eat three meals on Shabbos, and this is in parallel to the verse that says אכלהו היום כי היום שבת לה' היום לא תמצאהו בשדה "And Moshe said 'Eat that [manna] today; for today is the Shabbat unto Hashem; today you shall not find it [the manna] in the field.'" And three times "today" is written in this verse. Our Sages said: 'Any Jew who upholds the Three Meals on Shabbos will be saved from Three Calamities - from birthpangs of Mashiach, from judgments of Gehinom, and from the War of Gog and Magog.'
Additionally, the Mishnah Berurah says that we are even obligated to help support poor people to the extent that they have the Third Meal on Shabbos (and not just enough for one meal and night and one meal at day).
The Mechaber writes that
"A wise person will not fill his stomach in the Morning Meal, so that there will be room for the Third Meal."
How to Fullfill Third Meal:
The Third Meal can be fulfilled with a drop more than a kabeitza of bread. (Less than a kabeitza is considered a snack and not a meal). Other opinions hold that baked goods in the amount needed to say mezonos is acceptable. If there is no mezonos, then fish and or meat is acceptable. If there is no fish or meat, then fruits are acceptable. But really one should eat bread, with lechem mishnah if possible.
If there is only one lechem mishnah, one person can say hamotzi with the lechem mishnah and have in mind everyone else, if everyone else also has in mind that he is saying hamotzi with the lechem mishnah for them.