If one does not have bread for se’uda shlishit or finds it difficult to eat bread, be-di’avad he may fulfill his obligation by eating mezonot. Although one may not use mezonot for the first and second meals (SA 274:4), when it comes to se’uda shlishit, some maintain that the primary purpose of the meal is to increase one’s delight, not to reach satiety, so one is not obligated to eat bread specifically. Therefore, be-di’avad one fulfills the obligation with mezonot. If he does not have mezonot either, or cannot eat them, he should eat meat or fish. If one has no meat or fish, he should eat fruit, preferably cooked, since cooked fruit is considered more akin to a proper meal (SA 291:5).
Ideally one should plan his eating so that he will have an appetite for se’uda shlishit. If it turns out that he is eating se’uda shlishit not long after lunch, he should eat less at lunch so that he will have an appetite for se’uda shlishit. If one was not careful about this and as a result is full when it is time for se’uda shlishit, he may eat a bit more than a keveitza of bread and thereby fulfill his obligation. Be-di’avad, he may even eat only a kezayit of bread. If a kezayit of bread or other foods is still too much and would cause him grief, he has lost this mitzva opportunity (SA 291:1; MB ad loc. 2).
According to Rambam, one must make a berakha over wine during se’uda shlishit. Some understand this to mean that just as one makes kiddush before lunch, similarly he must make kiddush before se’uda shlishit (Tur). However, in practice the mitzva of kiddush is once by night and once by day, and there is no mitzva to make kiddush over wine at se’uda shlishit (SA 291:4). Others say that Rambam is merely ruling that there is a mitzva to drink wine at se’uda shlishit in order to make Shabbat more delightful. Indeed, several Aĥaronim write that it is best to beautify the mitzva by having wine at se’uda shlishit.
Se’uda shlishit must begin before shki’a. As long as one recited “ha-motzi” before shki’a and began the meal, he may continue to eat even well after tzeit. However, if one was not eating bread but mezonot, fruits, or vegetables, or if he was drinking, then once shki’a arrives he must stop, because these foods do not render this a proper meal; since the time for havdala has already arrived, one may not eat or drink (SA 299:1; MB ad loc. 2; AHS ad loc. 3-5; below 8:8).
One who did not manage to begin se’uda shlishit before shki’a may start eating up to 13.5 minutes after shki’a (and may continue eating well beyond nightfall). But later than that, he should not eat se’uda shlishit.
If a bride, a groom, and a minyan, were present at se’uda shlishit, Sheva Berakhot are recited at the conclusion of the meal. The person who leads the zimun, the bride, and the groom all drink from the wine after Birkat Ha-mazon even if it is past tzeit and they have not yet made havdala, because drinking this wine is a continuation of their meal (See below 8:8 where we record that some are accustomed to drink wine from the kos shel berakha even without a bride and groom present).