There is a custom to say Birkas Hamozohn holding a cup of wine, which is called "Kohs shel brocha". This is done on Shabbos and Yom Tov, at a wedding, or at a meal where a "minyan" (ten) of men are eating together. Even on these occasions, when one person "bentches" (says the prayer of Birkas Hamozohn) by himself, the custom is not to "bentch" over a cup of wine. (Mishnah B'rurah l82:l6) If three people eat together, they are not required to "bentch" over a cup of wine. However, if they do so, it is considered as if they did a "mitzvah" in the best possible way. (Mishnah B'rurah 182:4)
During the third meal of Shabbos, the custom may be different depending on the time we begin to "bentch". If Birkas Hamozohn was said before the end of Shabbos, then he may drink from the "Kohs shel brocha" even if he does not usually "bentch" over a cup of wine. (Mishnah B'rurah 299:14) However, if Shabbos has already ended, he should leave the "Kohs shel brocha" in the place where he ate the third meal, "bentch", and return to make "Havdollah" over this cup of wine. This way it is also counted as the "Kohs shel brocha". (Riv’vohs Ephraiyim 2:117, according to Rav Moshe Feinstein zt"l)
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