Wine from Se'uda Shlishit Birkat HaMazon

If you recite birkat ha'mazon after se'uda shlishit over a cup of wine, you may only drink the wine if the meal ended before sunset.  
Note Wine from birkat ha'mazon of se'uda shlishit that ended after sunset may be used for havdala EXCEPT if the meal was a sheva brachot meal.
Reason The bridegroom, bride, and leader may drink the wine--and one of them must drink the wine!--as part of the seven blessings, even though they were recited after sunset.