If sheva brachot occur at a seudah shlishit which finishes after sunset, is one allowed to drink the wine even though it is prior to making havdala? If yes, who specifically would be allowed to drink it? E.g. what about someone who was not the leader or one of the newlyweds, but just wants some for a segula?
Sh'miras Shabas K'hilchasah 59:17 reads as follows. The translation is my own and slightly free, and you should not rely on it; in any event, as always, CYLOR with any practical questions.
S'uda sh'lishis that they set up for a bridegroom within the week of sheva b'rachos, which extends until sunset or thereafter: So long as tzes hakochavim has not arrived, they say the b'rachos on two cups as usual, one for birkas hamazon and the other for sheva b'rachos.
And even if the meal ended only after tzes hakochavim, the custom is to say sheva b'rachos and to drink from the cup of birkas hamazon and [from the cup] of sheva b'rachos: it's permissible for the bride and bridegroom and all those present to drink from these cups. But some have the custom to say birkas hamazon and sheva b'rachos on two cups as usual but not to say the b'racha "hagafen" on either cup, and to leave the cups until after maariv and then to say havdala on the birkas hamazon cup, and then to taste from the sheva b'rachos cup as usual, namely that they mix the two cups and give it to the bridegroom, bride, and all those present.
He notes in footnote 65 that Rabbi Moshe Feinstein is quoted by Rabbi Efrayim Greenblatt as having said that the bride and bridegroom should drink from the cups but no one else. [The way the SSK quotes him, it sounds like he means this for the latter case (that s'uda sh'lishis extends past tzes hakochavim), but I haven't checked the RE.]
See also footnote 66.
(That's it for the SSK, but note that I seem to recall hearing from Rabbi Yisrael Reisman (Brooklyn, New York) the idea that the bridegroom lead birkas hamazon and drink immediately afterward. I don't recall whether that was his advice or something he had seen or what: I merely recall his mentioning the idea. It would seem to fit in with the discussion above: according to Rav Moshe, apparently, the leader of birkas hamazon shouldn't drink unless he's the bridegroom, whereas according to some others the leader can drink anyway, so this removes some doubt as to what to do.)
Regarding drinking the wine after reciting Birkat HaMazon over a cup of wine at Seudah Shelishit if it’s already past sunset, some Ashkenazim are stringent not to. However, assuming Shabbat is not the final day of Sheva Berachot, then the Tzitz Eliezer (vol. 10 Siman 45) argues that even Ashkenazim who usually refrain from drinking may do so in the context of Sheva Berachot.
Rav Moshe Shternbuch cites how Rav Yitzchak Ze’ev Soloveitchik would not either, and testified that his father, Rav Chaim Soloveitchik was also stringent, but his grandfather, Rav Refael Shapiro was not (Teshuvot veHanhagot vol. 1 Siman 749).
At the same time, Rav Moshe Feinstein highly discourages anyone but the Chattan and Kallah from drinking in such a situation (Iggerot Moshe, Orach Chaim vol. 4 Siman 69), as does Rav Mordechai Willig. See also Emmet LeYaakov (Kamenetsky, Siman 62 fn 29).