- The earliest time to fulfill Seudat Shelishit is from 6 and a 1/2 hours (Shaot Zmaniot) into the day. If one began before the time and continued and ate at least a Kezayit after the time he does fulfill his obligation for seuda shlishit.
- The Minhag is to eat Seudat Shelishit between Mincha and Maariv/Arvit. [1] If one is unable to have it after Mincha one should have it before mincha. [2]
- Although it is forbidden to begin eating after sunset until one says Havdalah, and it is definitely preferable to eat before sunset, some poskim give extra time to begin the meal if one hasn't yet eaten seuda shlishit.
- Rama 291:2. Yalkut Yosef shabbat volume 1 page 402-403 Rabbeinu Tam quoted by the Tosfot Pesachim 105a "vihani milei", as well as the Rosh Shabbat 10:13 say that one should eat before saying Mincha because it is forbidden to drink water between Mincha and Arvit on shabbat because drinking between Mincha and Arvit could cause harm to the souls that leave after shabbat. Rambam Hilchot Shabbat 30:10 writes that it is best to eat it after saying Mincha and the hagahot maimoniot hilchot shabbat 30:20 agrees and quotes a different version of rabbeinu tam. The Rama quotes both opinions and says that the minhag is to do like the Rambam. The Magen Avraham 291:5 says this is because of the prohibition to eat before davening mincha. Aruch Hashulchan 291:4 also says the minhag is that way also.
- Aruch HaShulchan 291:4. Mishna Brurah 291:11 says to make sure to eat before the time of Mincha ketana arrives. Yalkut Yosef Shabbat volume 1 page 402 says also that you fulfill your obligation but should try to get somebody to remind you to say Mincha if you are going to eat before saying it.
(ט) חַיָּב אָדָם לֶאֱכל שָׁלֹשׁ סְעֻדּוֹת בְּשַׁבָּת אַחַת עַרְבִית וְאַחַת שַׁחֲרִית וְאַחַת בְּמִנְחָה. ... וְצָרִיךְ לִקְבֹּעַ כָּל סְעֻדָּה מִשְּׁלָשְׁתָּן עַל הַיַּיִן וְלִבְצֹעַ עַל שְׁתֵּי כִּכָּרוֹת. וְכֵן בְּיָמִים טוֹבִים:
(י) אֲכִילַת בָּשָׂר וּשְׁתִיַּת יַיִן בְּשַׁבָּת עֹנֶג הוּא לוֹ. וְהוּא שֶׁהָיְתָה יָדוֹ מַשֶּׂגֶת. וְאָסוּר לִקְבֹּעַ סְעֻדָּה עַל הַיַּיִן בְּשַׁבָּת וּבְיָמִים טוֹבִים בִּשְׁעַת בֵּית הַמִּדְרָשׁ. אֶלָּא כָּךְ הָיָה מִנְהַג הַצַּדִּיקִים הָרִאשׁוֹנִים מִתְפַּלֵּל אָדָם בְּשַׁבָּת שַׁחֲרִית וּמוּסָף בְּבֵית הַכְּנֶסֶת וְיָבוֹא לְבֵיתוֹ וְיִסְעֹד סְעֻדָּה שְׁנִיָּה וְיֵלֵךְ לְבֵית הַמִּדְרָשׁ יִקְרָא וְיִשְׁנֶה עַד הַמִּנְחָה וְיִתְפַּלֵּל מִנְחָה וְאַחַר כָּךְ יִקְבַּע סְעֵדָּה שְׁלִישִׁית עַל הַיַּיִן וְיֹאכַל וְיִשְׁתֶּה עַד מוֹצָאֵי שַׁבָּת:
(9) One should eat three meals on the Sabbath: one in the evening, one in the morning, and one in the afternoon. ... And he is required to fix each meal of the three on wine and to break [bread] on two loaves. And likewise on holidays.
(10) The eating of meat and the drinking of wine on Shabbat is considered a delight (oneg) for him; and that is so long as he can afford it. But it is forbidden to fix a meal on wine on Shabbat and festivals at the time of [a] study hall [session]. Rather this was the custom of the early righteous ones: They would pray the morning and additional service in the synagogue, then go to their home and eat the second meal, and then return to the study hall to read (from the Bible) and study (from the Mishnah), and pray the afternoon service; and afterwards fix a meal on wine and eat and drink until the conclusion of Shabbat.
Rambam famously mandated the use of wine for all three meals of Shabbat. This seems to be the reason that he does not want you to eat/drink at Se'udah Shelishit before your afternoon learning seder and Mincha.
Perhaps one could infer from his words that drinking is done more at Se'udah Shelishit than at lunch for this very reason. It is likely that he expects you to delay Arvit until you are ready to pray in a completely sober state - see next source - although I am not sure what happens for Birkat Hammazon. About the latter point, Rambam makes no mention of not bensching after one has drank a lot of wine. Perhaps he would say it is fine to bensch after drinking a revi'it, unlike set prayer, but as long as you are in a fit state of mind and in no way drunk.
(טו) כַּוָּנַת הַלֵּב כֵּיצַד. כָּל תְּפִלָּה שֶׁאֵינָהּ בְּכַוָּנָה אֵינָהּ תְּפִלָּה. וְאִם הִתְפַּלֵּל בְּלֹא כַּוָּנָה חוֹזֵר וּמִתְפַּלֵּל בְּכַוָּנָה. מָצָא דַּעְתּוֹ מְשֻׁבֶּשֶׁת וְלִבּוֹ טָרוּד אָסוּר לוֹ לְהִתְפַּלֵּל עַד שֶׁתִּתְיַשֵּׁב דַּעְתּוֹ. לְפִיכָךְ הַבָּא מִן הַדֶּרֶךְ וְהוּא עָיֵף אוֹ מֵצֵר אָסוּר לוֹ לְהִתְפַּלֵּל עַד שֶׁתִּתְיַשֵּׁב דַּעְתּוֹ. אָמְרוּ חֲכָמִים יִשְׁהֶה שְׁלֹשָׁה יָמִים עַד שֶׁיָּנוּחַ וְתִתְקָרֵר דַּעְתּוֹ וְאַחַר כָּךְ יִתְפַּלֵּל:
(טז) כֵּיצַד הִיא הַכַּוָּנָה. שֶׁיְּפַנֶּה אֶת לִבּוֹ מִכָּל הַמַּחֲשָׁבוֹת וְיִרְאֶה עַצְמוֹ כְּאִלּוּ הוּא עוֹמֵד לִפְנֵי הַשְּׁכִינָה. לְפִיכָךְ צָרִיךְ לֵישֵׁב מְעַט קֹדֶם הַתְּפִלָּה כְּדֵי לְכַוֵּן אֶת לִבּוֹ וְאַחַר כָּךְ יִתְפַּלֵּל בְּנַחַת וּבְתַחֲנוּנִים וְלֹא יַעֲשֶׂה תְּפִלָּתוֹ כְּמִי שֶׁהָיָה נוֹשֵׂא מַשּׂאוֹי וּמַשְׁלִיכוֹ וְהוֹלֵךְ לוֹ. לְפִיכָךְ צָרִיךְ לֵישֵׁב מְעַט אַחַר הַתְּפִלָּה וְאַחַר כָּךְ יִפָּטֵר. חֲסִידִים הָרִאשׁוֹנִים הָיוּ שׁוֹהִין שָׁעָה אַחַת קֹדֶם תְּפִלָּה וְשָׁעָה אַחַת לְאַחַר תְּפִלָּה וּמַאֲרִיכִין בִּתְפִלָּה שָׁעָה:
(יז) שִׁכּוֹר אַל יִתְפַּלֵּל מִפְּנֵי שֶׁאֵין לוֹ כַּוָּנָה. וְאִם הִתְפַּלֵּל תְּפִלָּתוֹ תּוֹעֵבָה. לְפִיכָךְ חוֹזֵר וּמִתְפַּלֵּל כְּשֶׁיִּתְרוֹקֵן מִשִּׁכְרוּתוֹ. שָׁתוּי אַל יִתְפַּלֵּל וְאִם הִתְפַּלֵּל תְּפִלָּתוֹ תְּפִלָּה. אֵי זֶה שִׁכּוֹר זֶה שֶׁאֵינוֹ יָכוֹל לְדַבֵּר לִפְנֵי הַמֶּלֶךְ. שָׁתוּי יָכוֹל לְדַבֵּר בִּפְנֵי הַמֶּלֶךְ וְאֵינוֹ מִשְׁתַּבֵּשׁ. אַף עַל פִּי כֵן הוֹאִיל וְשָׁתָה רְבִיעִית יַיִן לֹא יִתְפַּלֵּל עַד שֶׁיָּסִיר יֵינוֹ מֵעָלָיו
(15) Concentration of the mind—how is this condition [to be fulfilled?] Any prayer uttered without mental concentration is not prayer. If a service has been recited without such concentration, it must be recited again devoutly. A person finds that his thoughts are confused and his mind is distracted: He may not pray till he has recovered his mental composure. Hence, on returning from a journey or if one is weary or distressed, it is forbidden to pray till his mind is composed. The sages said that he should wait three days till he is rested and his mind is calm, and then he recites the prayers.
(16) What is to be understood by concentration of the mind? The mind should be freed from all extraneous thoughts and the one who prays should realize that he is standing before the Divine Presence. He should therefore sit awhile before beginning his prayers, so as to concentrate his mind, and then pray in gentle tones, beseechingly, and not regard the service as a burden which he is carrying and which he will cast off and proceed on his way. He should, accordingly, also sit awhile, after concluding the prayers, and then leave. The ancient saints were wont to pause and meditate one hour before the service, one hour after the service and take one hour in its recital.
(17) An intoxicated person must not pray, because he cannot concentrate. If he prays, his prayer is an abomination. He must therefore recite the prayers again after he has recovered from his intoxication. A person under the influence of drink should not pray while in that condition. But if he has recited the service, it is regarded as prayer. A drunkard is one who is unable to speak in the royal presence. A person under the influence of drink is one who can speak in the presence of a king without committing error. Yet having drunk, if only a revi'it (86 ml) of wine, one is not to pray till he is rid of the effect of the wine he has taken.
(ב) זמנה משיגיע זמן המנחה דהיינו משש שעות ומחצה ולמעלה ואם עשאה קודם לכן לא קיים מצות סעודה שלישית: הגה י"א דאסור לשתות מים בין מנחה למעריב בשבת דאז חוזרים הנשמות לגיהנם וע"כ אין לאכול סעודה שלישית בין מנחה למעריב אלא יאכל אותה קודם מנחה (תוס' והרא"ש ומרדכי פ' ע"פ) וי"א דיותר טוב להתפלל מנחה תחלה (רמב"ם וטור והגהות מרדכי והגהות מיי' פ"ל ואגור) וכן נוהגים לכתחלה בכל מדינות אלו מ"מ אין לשתות מים מן הנהרות אבל בבית שרי וכ"ש שאר משקין דשרי (הגהות מיי' פ"ל) וי"א דאין אסור אלא תוך י"ב חודש של אביו ואמו (אגודה ומרדכי פ' ע"פ) וי"א דאיסור זה של שתיית מים אינו רק בע"ש (תוס' והרא"ש ומרדכי בשם רבי משולם והגהות מיי'):
(2) 2. The time [of this meal] is from the time of minchah, which is from the sixth and a half hour onward. And if he does this earlier, he has not fulfilled the mitzvah of the third meal. RAMA: There are those who say that it is prohibited to drink water from Minhah until Maariv on Shabbat for that is when the souls return to Gehinom. And therefore one should not eat the third meal between Minhah and Maariv. Instead he should eat it before Minhah (Tosafot; the Rosh and Mordecai, chapter ten of Pesahim). And there are those who say that it is is preferable to pray Minhah first (Rambam, Tur, Hagahot Mordecai, Hagahot Maimoni, chapter 30 and Agur). And this is our preferred custom in all of these lands. And in any case one should not drink water from the rivers, but it is permissible in one's house. And all the more so other liquids are permissible (Hagahot Maimoni, chapter 30). And there are those who say that this is prohibited only during the twelve months after one's mother or father died (Agudah and Mordecai, chapter 10 of Pesahim). And there are those who say that this prohibition of drinking water is only on Friday eve (Tosafot, Rosh, and Mordecai in the name of R. Meshulam, and Hagahot Maimoni.)
Shulchan Aruch just has the time of Mincha as the time window for Se'udah Shelishit. Rema on the other hand quotes Rambam and others that one should eat after Mincha and he comments that it is the ideal practice. This is despite an opinion that water should not be drank (and therefore the meal should not be eaten) after Mincha on Shabbat, which is less applicable (and authoritative?) than the other view.
ד זמן סעודה שלישית הוא משיגיע זמן מנחה, דהיינו משש שעות ומחצה ולמעלה. ויתפלל מנחה קודם שיאכל, או שיעמיד שומר להזכירו להתפלל מנחה. ואם עשאה קודם לכן יחזור ויאכל כשיגיע זמנה. ואם התחיל באכילתו קודם זמן מנחה, ותוך כדי אכילתו הגיע זמן מנחה, והמשיך באכילתו, ואכל כזית אחר שש שעות ומחצה, יצא ידי חובת סעודה שלישית, דבזה לא אזלינן בתר תחלת הסעודה. [ילקו''י שבת כרך א' עמוד תב].
ה נכון יותר לאכול סעודה שלישית אחר שהתפלל מנחה, וכן המנהג. ואם אכל קודם מנחה, ואי אפשר לו לחזור ולאכול אחר מנחה, יצא ידי חובת סעודה שלישית. וכן במקום שקשה לו לאכול אחר תפלת מנחה, או שאין שם מנין לתפלת מנחה, יכול לאכול אפילו לכתחלה קודם תפלת מנחה, ובלבד שיגיע זמן מנחה גדולה. ולכן במושב שיש שם בית כנסת אחת, ובגלל קושי הדרך מתפללים מנחה וערבית במוצ''ש סמוכים זה לזה, יקדימו לאכול קודם מנחה. אלא שנכון יותר להנהיג להתפלל מנחה גדולה, כדי שיוכלו לאכול אחר תפלת מנחה בצבור, בפרט שלדעת האר''י ז''ל, זמן סעודה שלישית הוא בדוקא אחר שהתפלל מנחה. [שם עמוד תג].
כֵּיצַד חוֹבַת סְעֵדָּה זוֹ. שֶׁיֹּאכַל בָּשָׂר וִיתַקֵּן סְעֵדָּה נָאָה כְּפִי אֲשֶׁר תִּמְצָא יָדוֹ. וְשׁוֹתֶה יַיִן עַד שֶׁיִּשְׁתַּכֵּר וְיֵרָדֵם בְּשִׁכְרוּתוֹ.
How is the obligation of this meal? That one eat meat and prepare as pleasing a meal as his hand can [afford]. And he should drink wine until he becomes intoxicated and falls asleep from his intoxication
On this occasion, he has to actually get drunk to the point of unconsciousness. Does that mean he does not Bensch till later?