#Although it is forbidden to begin eating after sunset until one says havdala, <ref> Shulchan Aruch 291:1, Rif Pesachim 21b. </ref> and it is definitely preferable to eat before sunset, <ref> Mishna Berura 299:1, Magen Avraham 299:1. See Shmirat Shabbat Kihilchita chapter 56 note 15 where he says that the idea that one should eat at least a kezayit of bread after tzet hakochavim on friday night quoted in Mishna Berurah 267:5, because there is an opinion that says you cannot fulfill a seudat shabbat during tosefet shabbat, should carry over to seudat shlishit, and therefore to satisfy this opinion one should be stringent and eat prior to sunset. </ref> some poskim give extra time to begin the meal if one hasn't yet eaten seuda shlishit. <ref> Shaar Hatziyun 299:2 gives two reasons for this. First, he says one can rely on the opinion of the Taz 299:1 who thinks that the Rosh Pesachim 12:10 allows eating until nightfall since there is a potential fulfillment of a mitzva. Second, the potential fulfillment of the mitzvah of seudah shlishit should override the potential prohibition of eating during bein hashimashot.
*Mishna Berura 299:1 says that if one hasn't yet eaten the meal or is really starving then one can still eat until a half hour before tzet hakovachim. R. Avraham Chaim Naeh in Ketzot HaShulchan (chapter 94 note 6), notes that Mishna Berurah's ruling that one may eat until a half hour prior to nightfall is following the opinion of Rabbeinu Tam that nightfall is seventy-two minutes after sundown. However, according to the Vilna Gaon, nightfall can be within one half hour of sunset. If so, one who follows the opinion of the Vilna Gaon would be prohibited from starting to eat one half hour before nightfall (of the Vilna Gaon) which is prior to sunset. Rav Shlomo Zalman Auerbach quoted in Shemirat Shabbat Kehilchata chapter 56 note 17 says that since even those who follow the opinion of the Vilna Gaon with regards to when to end shabbat will never actually recite Havdalah within one-half hour of sunset, they may start eating up until sundown.
*Pninei Halacha [[Shabbat]] vol. 1 page 130 quotes Rav Moshe Feinstein Iggerot Moshe 4:69:6 that one has until thirteen minutes and a half minutes after sunset. Yalkut Yosef shabbat 1 page 414
#If one began eating before sunset he may continue after. <ref> Shulchan Aruch 299:1, Aruch Hashulchan 299:3. Mishna Berura 291:2 says that having said the beracha is enough of a start to your meal to allow one to continue afterwards. He also quotes an opinion that this only applies during bein hashemashot but after tzet hakochavim one would not be allowed to continue. Rama says that the minhag is not so, rather one can continue even after tzet hakochavim, and the aruch hashulchan 299:4 says that is the minhag. </ref> However, this doesn't apply if one was only drinking not as part of a meal <ref> Shulchan Aruch 299:1 </ref> or to eating only mezonot or fruit, so one must stop if that is all that he is having. <ref> Aruch Hashulchan 299:5 </ref>