First, let us discuss the halacha regarding drinking a cup of wine for Birkat HaMazon at a regular se’uda shlishit that ends at night. The Magen Avraham[2] says that one who bentches over a cup of wine at se’uda shlishit may drink it before Havdala. Since the cup of wine is part of the bentching, drinking it in effect concludes the meal. Just as one can complete se’uda shlishit at night before Havdala, he can similarly drink the wine that is connected to the meal. However, the Magen Avraham adds the caveat that if one does not regularly recite Birkat HaMazon over wine, in a case when he does use wine, he should not drink it before Havdala. For such a person, the connection between the wine and the meal is insufficiently strong to justify drinking before Havdala.[3] Eliya Rabba[4] and Tosefet Shabbat[5] are not convinced that the fact that one does not use a cup regularly for Birkat HaMazon makes a difference in this regard, but poskim are reluctant to reject a ruling of the Magen Avraham without further indications.[6]
What should one do about sheva berachot at se’uda shlishit according to the Magen Avraham? On the one hand, since most of us are not vigilant to always bentch over a cup of wine, perhaps we should not drink the wine before Havdala. On the other hand, we always use a cup for sheva berachot. Given that our minhag is to use two cups of wine to serve the two purposes, should we distinguish between the two? Rav Shlomo Kluger[7] reasons that although logic dictates that it is permitted to drink the cup of sheva berachot but not the cup of bentching, doing that would diminish the status of the cup of Birkat HaMazon. Therefore, he suggests that one not drink from either cup before Havdala. The Eshel Avraham[8] argues that at sheva berachot, one should drink from the cup before Havdala because without the preceding beracha of Borei Pri HaGefen, the requisite seven berachot are lacking. Others say that if one always uses a cup for bentching at sheva berachot or at formal gatherings with a minyan, that is sufficient to enable the drinking from the cup.[9]
Assuming that the beracha of Borei Pri HaGefen is recited and the wine is drunk, further issues and various opinions come into play. First, who should drink the wine? Rav Moshe Feinstein[10] posits that there are different aspects of the cups of wine at sheva berachot. The fact that the chatan and kalla drink is connected to the very nature of sheva berachot, which requires a cup of wine; therefore, they should drink before Havdala. The reason why the person who bentches at sheva berachot drinks is related to the general matter of bentching over a cup. Since his drinking is not crucial, Rav Moshe posits that it is preferable that only the chatan and kalla drink the necessary cheeks-full amount of wine. Some propose just the opposite – that is, the one who makes the beracha should drink (as well as the kalla, since not all options maintain that women are obligated in Havdala), but the chatan should not. The latter distinction is tenuous.[11]
Some poskim prefer that those drinking should only take a sip,[12] as a sip may be permitted before Havdala and may be sufficient for drinking from the cup of sheva berachot. If the one who bentches, the chatan, and the kalla each has about an ounce of wine, additional opinions would be satisfied.[13] However, it is perfectly acceptable for at least the three people mentioned to drink as usual from the cup(s).[14] [15]