The daily Birkot Ha’Torah blessings, which we recite each morning to thank Hashem for the Misva of studying Torah, consists of several sections. It begins with the brief Beracha, "Baruch…Asher Kideshanu Be’misvotav Ve’sivanu Al Dibreh Torah," which is followed by the lengthier Beracha, "Ve’ha’areb Na…" Birkot Ha’Torah concludes with the Beracha of "Baruch Ata…Asher Bahar Banu…Baruch Ata Hashem Noten Ha’Torah."
The second of these sections – "Ve’ha’areb Na" – begins with "Ve" ("And"), which clearly indicates that it continues the previous blessing. As such, one might think that "Ve’ha’areb Na" does not begin a new Beracha, but is rather just the continuation of the first Beracha. Although this is, in fact, the opinion of some Poskim, Sephardic tradition follows the view that Birkot Ha’Torah consists of three separate Berachot, and "Ve’ha’areb Na" begins a new Beracha. Therefore, as Hacham David Yosef rules in Halacha Berura (vol. 3, p. 380), if one hears his fellow reciting Birkot Ha’Torah, he answers "Amen" three times – after "Al Dibreh Torah," after the conclusion of "Ve’ha’areb Na" (meaning, after "He’melamed Torah Le’amo Yisrael"), and after "Baruch Ata Hashem Noten Ha’Torah."
Summary: The various sections that comprise Birkot Ha’Torah are considered three separate Berachot. Therefore, if one hears his fellow reciting Birkot Ha’Torah, he answers "Amen" three times – after "Al Dibreh Torah," after the conclusion of "Ve’ha’areb Na" (meaning, after "He’melamed Torah Le’amo Yisrael"), and after "Baruch Ata Hashem Noten Ha’Torah."