Be very careful to fulfill the third meal. It saves from 3 punishments: the birthpangs of the Messianic era, from Gehinom, and from the war of Gog and Magog. (ShA 1 – MB 1)
How much bread must you eat at each seuda? L’chatchila, more than a k’baitza. Some hold you can get away with a k’zayis. If you can’t eat even that, don’t pain yourself — it won't even be a mitzva. However, a chacham plans ahead not to eat too much in the morning seuda. (ShA 1 – MB 2, 4)
The time for Shalosh Seudos is from Mincha Gedola (6 ½ hours into the day). L’chatchila you should daaven Mincha beforehand. If it will be difficult to hold Shalosh Seudos after Mincha, you may hold it before you daaven as long as it is before Mincha Ketana. (ShA 2 – MB 11)
If you think you won’t be able to eat Shalosh Seudos after daavening Mincha (e.g. you are full and it is a short Shabbos), you are allowed to interrupt the morning seuda in the middle, bentch, get up and take a walk, wash again for Hamotzi, and eat the rest of the meal for sake of Shalosh Seudos. Be sure that the second half of the meal is after chatzos. (ShA 3 – MB 14)
If eating Shalosh Seudos after daavening Mincha will not be an achilla gasoh, do not split your morning meal into two — some say it is causing unnecessary berachos. (Rama 3 – MB 15)
When Yom Tov falls out on Motzo'ay Shabbos do not hold a big Shalosh Seudos at the end of the day, like normal. It will ruin your appetite for the Yom Tov night meal. However, to have a bit more than a k’baytza of bread is permissible. (MB 17 – BeH “achar”)
To postpone the morning seuda until after chatzos is a transgression of oneg Shabbos. (MB 17)
You don’t need to say Kiddush before Shalosh Seudos, unless you weren’t able to say Kiddush before the morning seuda. (ShA 4 – ShaH 9)
L’chatchila, you should have Lechem Mishneh for Shalosh Seudos. (MB 21)
It is good to beautify the meal with a beracha on wine during the meal. (MB 21)
Shalosh Seudos is a much more chashuv meal than Melaveh Malka. (BeH “ul’fachos”)
It’s a mitzva l’chatchila to have many delicacies to eat for Shalosh Seudos. (MB 22)
If you don’t eat bread for Shalosh Seudos, there is a dispute amongst the poskim if you can fulfill it with other foods. Some say it must at least consist of one of the five grains. Others are lenient to eat condiments like meat and fish. The most lenient opinion allows even fruit for Shalosh Seudos. But all agree that bread and Lechem Mishneh is l’chatchila. (ShA 5 – MB 23)
When Erev Pesach falls out on Shabbos, fulfill Shalosh Seudos with types of fried matza crumbs, meat, fish or fruit. (MB 25)
Women are just as obligated in Shalosh Seudos and Lechem Mishneh as men are. (ShA 6 – BeH “noshim”)