Question: Our shul davens Mincha on Shabbos about an hour before Shabbos ends. By the time I get home, it is after shekia. Is that too late to start eating seuda shelishis?
Answer: The Shulchan Aruch (OC 299:1) writes that one shouldn’t start eating when it gets dark on Shabbos afternoon until havdala. If one had started eating a meal before this time, one can continue eating (See Aruch Hashulchan OC 299:3).
The Mishna Berura (299:1) writes that while it is preferable to eat before shekia, if one hasn’t yet eaten seuda shelishis, then one should eat up to half an hour before tzeis hakochavim. Among other reasons, he explains (Shaar Hatziyun 299:2) that we are not accustomed to be so strict about shekia, especially when there’s a mitzva to eat seuda shelishis.
R’ Moshe Sternbuch (Teshuvos Vehanhagos 2:203) writes that while the Chazon Ish was particular not to eat after shekia, nonetheless, most poskim allow one to begin eating seuda shelishis a few minutes after shekia (See Shemiras Shabbos Kehilchasa 56:4; Rivevos Ephraim 1:264).In conclusion, it is preferable to begin eating seuda shelishis before shekia, though it is justifiable to begin a few minutes afterwards if necessary.