What is the proper time to begin Seudah Shlishit and to recite Birkat Ha’mazon on Shabat?

It is proper to begin Seudah Shelishit (the third Shabat meal) before sundown on Shabat. One may begin the meal even just several minutes before sundown and continue eating beyond sundown. If one was unable to begin Seudah Shelishit before sunset, then he may begin the meal even after sundown, within the period of Ben Ha’shemashot, which extends for 13.5 minutes after sundown.

It is proper to recite Birkat Hamazon after Seudah Shelishit over a cup of wine. After completing Birkat Hamazon, the one who led Birkat Hamazon may drink the wine, and, according to Ha’Rav Ovadiyah Yosef (Shlit”a), he may also give the cup to other people at the table so they have a sip of the wine. There are chachamim who maintain that one should not drink the cup of wine used for Birkat Hamazon at Seudah Shelishit, as it should rather be used for Havdalah. The Sefardik minhag, however, does not follow this view, and it is therefore 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.