It is proper to begin Se置da Shelishit (the third Shabbat meal) before sundown on Shabbat. One may begin the meal even just several minutes before sundown and continue eating beyond sundown.
If one was unable to begin Se置da Shelishit before sunset, then he may begin the meal even after sundown, within the period of Ben Ha痴hemashot, which extends for 13.5 minutes after sundown.
It is proper to recite Birkat Hamazon after Se置da Shelishit over a cup of wine. After completing Birkat Hamazon, the one who led Birkat Hamazon may drink the wine, and, according to Hacham Ovadia Yosef, he may also give the cup to other people at the table so they have a sip of the wine. There are authorities who maintain that one should not drink the cup of wine used for Birkat Hamazon at Se置da Shelishit, as it should rather be used for Habdala. Sephardic practice, however, does not follow this view, and it is thus permissible for the one who led Birkat Hamazon to drink the cup of wine, and for the others at the meal to sip some of the wine, after Birkat Hamazon.
