The angels blesses Sarah and sends a Kos Shel Bracha (Rashi Breshit 18:9 based on Bava Metzia 87a). the question is how was there a Kos Shel Bracha if there was no bread as it says Sarah became a Niddah (Rashi Breshit 18:8 based on Bava Metzia 87a) and therefore did not serve the bread?
We learn she suddenly became Nidah in Bava Metzia 87 (Rashi). From the Torah Temima we learn that the Kos Shel Bracha doesn't have to be of the Birkat Hamazon (Benching?). (He brings an opinion of Rabbi Akiva at Brachot 44a that whatever you eat if this is his "mezono" you mevarech with wine- but we don't follow this opinion). So what they did is that first they drank and only afterwards they sent the glass to the Nidah (Sarah).
ANSWER FROM Divrei Chaim BLOG
The Lev Simcha explains that the kos shel bracha is the kos from bentching over the future seudas Livyasam. Chazal (Pesacim 119b) tell us that the kos shel bracha will be passed from tzadik to tzadik, from Avraham to Yitzchak to Ya'akov etc. and each one will have a reason why he should not lead bentching. Avraham will decline because he had Yishmael; Yitzchak will decline because he had Eisav, etc. Finally, the kos will get to David haMelech and he will accept it. The tzenius of Sarah which provided the justification for the derasha of "Moavi v'lo Moavis," paving the way for the acceptance of David haMelech, is rewarded with a taste of the future kos shel bracha that only David haMelech can accept at that seudah of Livyasan in the time of future geulah.
The Maharil Diskin answers that the gemara in Berachos 35b asks if one should bentch if he makes wine the basis of his meal, and answers that we have to wait for Eliyohu (i.e. a prophet) to tell us whether wine can really serve as a basis for a meal. Therefore, since Avrohom was a prophet he determined that wine could be considered as the basis of a meal, and thus they bentched on a meal based on wine and therefore there was a Kos Shel Berachah.