shlosh se'im - three measures
The three measures described are known as a single measure called efah. Our Sages in Menachot 76 say that three measures called sa-ah constitute the measure known as efah.
Seeing that the Torah usually economizes with its words, why did it not write efah, a single word, instead of writing shlosh se'im in two words? It is possible that the reason Avraham spoke in terms of three measures was due to etiquette. He meant to assure each one of the three angels of a whole measure for him exclusively, which would be made into cake. This would ensure that each of them received equal portions and that there would not be any kind of envy between one and the other during the meal (Megillah 12).
He may have had yet another consideration. He wanted to attract a crowd in order to demonstrate to them to what lengths he and Sarah would go in order to entertain uninvited strangers. It was similar to what our Sages recommended we should do in order to welcome the Shabbat (Shabbat 119) where we find a discussion on the manner in which the Shabbat is to be welcomed.
The Talmud relates that Rav Nachman bar Yitzchak was constantly busy coming and going on the Shabbat eve (making a commotion). When asked why he was in such an uproar on account of the Shabbat, he replied that if the two leading scholars of his time Rabbi Ami and Rabbi Assi were to be his guests would he not go out of his way to honor them? The Shabbat does not deserve any less honor.
Another thing we can learn from this detail the Torah provides us with when mentioning the amount of flour used by Sarah to bake cakes for her guests, is that Avraham was of a generous disposition. Had he used the expression efah, the impression created would have been that he wanted to limit the amount of flour to be used, i.e. "one measure, not more." In order to prevent such an impression being created the Torah decided to write an extra word in order to preserve Avraham's image as a generous person.