Chaim asks:
Do we use on cup one wine at Seudat shelshit and havdallah or two cups?
Chaim , Florida
The Kollel replies:
Dear Chaim,
If you finish Se'udah Shlishis before sunset, there's no problem to drink the wine, and Havdalah gets a new cup. If it's already dark, the cup cannot be drunk after Birkas ha'Mazon (except those who always bentch with a cup). (There is a problem already once sunset arrives.) It is proper to use the same cup for Havdalah because that is called eventually drinking the cup of Birkas ha'Mazon. (See Zos ha'Berachah of Rav Mandelbaum, p. 137, about Se'udah Shelishis.)
All the best,
Reuven Weiner