The term "Beracha Me’en Shalosh" refers to the Beracha Aharona that one must recite after eating certain foods. There is a common misconception that this Beracha is called "Me’en Shalosh" (literally, "similar to three") because there are three variations of the text of this Beracha. After eating grain products one recites, "Al Ha’mihya Ve’al Ha’chalkala"; after drinking wine one recites, "Al Ha’gefen Ve’al Peri Ha’gefen"; and after eating certain fruits one recites, "Al Ha’etz Ve’al Peri Ha’etz." But this is not the real reason why we call this Beracha "Me’en Shalosh." It is so named because it summarizes the major themes of Birkat Ha’mazon, which consists of three main Berachot: "Ha’zan," "Al Ha’aretz," and "Boneh Yerushalayim." Since the Beracha Aharona incorporates the themes of these three Berachot, we refer to it with the term "Me’en Shalosh."
In truth, Birkat Ha’mazon also contains a fourth Beracha – "Ha’tob Ve’hametib" – which was instituted later by the Sages. Only the first three Berachot are required by force of Torah obligation, while the fourth is recited by force of Rabbinic enactment. The conclusion of the Beracha Me’en Shalosh indeed makes reference to the theme of this fourth Beracha – "Ki Ata Tob U’metib La’kol…" We nevertheless call the Beracha "Me’en Shalosh," rather than "Me’en Arba" ("similar to four"), because the essential Birkat Ha’mazon text – that which is required by Torah law – consists of only three Berachot.
The Beracha Me’en Shalosh resembles Birkat Ha’mazon also with regard to the requirement to add special insertions on Shabbat, Rosh Hodesh and Yom Tob. Just as we add "Reseh" in Birkat Ha’mazon on Shabbat, and "Ya’ale Ve’yabo" on Rosh Hodesh and Yom Tob, we similarly add a brief reference to Shabbat, Rosh Hodesh and Yom Tob in Me’en Shalosh on these occasions. In fact, these insertions are added in the Beracha Me’en Shalosh at the point which corresponds to the point in Birkat Ha’mazon where we insert "Reseh" and "Ya’ale Va’yabo." In Birkat Ha’mazon, we add these insertions after the Beracha of "Boneh Yerushalyim," before the fourth Beracha of "Ha’tob Ve’hametib," and we likewise make the insertions in the Me’en Shalosh just before the phrase, "Ki Ata Tob U’metib," which corresponds to the fourth Beracha of Birkat Ha’mazon.
An interesting question arises in a case where a person reciting Me’en Shalosh forgot to add the required insertion, and realized his mistake before he completed the Beracha, but after he recited the words, "Ki Ata Tob U’metib." When it comes to Birkat Ha’mazon, if a person forgot to add "Reseh" or "Ya’aleh Ve’yabo" and remembered his mistake only after he began reciting the fourth Beracha, he may no longer correct his mistake. Depending on when this occurred, he will either be required to begin Birkat Ha’mazon anew, or he would simply continue reciting Birkat Ha’mazon. In all cases, however, he does not have the opportunity to return to "Reseh" or "Ya’ale Ve’yabo" once he began reciting the fourth Beracha, the Beracha of "Ha’tob Ve’hametib."
According to the Eliyahu Rabba (Rav Eliyahu Shapiro of Prague, 1660-1712), this rule applies to Me’en Shalosh, as well. Meaning, if a person forgot to recite the required insertion for Shabbat, Rosh Hodesh or Yom Tob, and he remembered his mistake only after he recited the words, "Ki Ata Tob U’metib," he may no longer correct his mistake. He simply continues reciting the Beracha, without adding the insertion. This is also the view of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Maseh.
Others, however, draw a distinction between Birkat Ha’mazon and Me’en Shalosh in this regard. In Birkat Ha’mazon, once a person begins the Beracha of "Ha’tob Ve’hametib," he may longer recite "Reseh" or "Ya’ale Ve’yabo" because he has begun a new Beracha, which constitutes a separate section of Birkat Ha’mazon. It is too late to recite "Reseh" or "Ya’ale Ve’yabo" at that point, because he is now in an entirely different section. Me’en Shalosh, however, constitutes a single Beracha. Although it contains all the themes discussed in Birkat Ha’mazon, these themes are all incorporated into a single Beracha. As such, so long as a person had not yet recited the words "Baruch Ata Hashem" in the conclusion of Me’en Shalosh, it is not too late for him to correct his mistake. This is the ruling of the Ma’amar Mordechi, and of Hacham Ovadia Yosef, in his work Halichot Olam, and this is, indeed, the Halacha. Therefore, a person who forgot to add the insertion for Shabbat, Rosh Hodesh or Yom Tob in Me’en Shalosh may add it later in Me’en Shalosh, so long as he has not yet recited "Baruch Ata Hashem" in the Beracha’s conclusion.
Summary: A person who forgot to add the insertion for Shabbat, Rosh Hodesh or Yom Tob in Me’en Shalosh may add it later in Me’en Shalosh, so long as he has not yet recited "Baruch Ata Hashem" in the Beracha’s conclusion. If he already recited "Baruch Ata Hashem," then he simply completes the Beracha. One who realized his mistake only after finishing the Beracha does not need to repeat it.
