Drinking from the Sheva Berachot cups at Seudah Shlishit

Hi,

We don't eat or drink after sunset on Shabbat, until Shabbat is over and one has recited Havdalah.

Therefore, where Seudah Shlishit is also a meal of Sheva Berachot and it ends after sunset, the person who recites birkat hamazon does not drink from the cup. However, the chatan and kallah do drink.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 4:69:1)

Have a great day,
Mordechai