Ha’Rav Ben Tzion Abba Shaul (a”h) addresses this question in his sefer Or L’tzion and he states, based on the ruling of the Ma’amar Mordechai, that one should, in fact, interrupt to answer to Kadish or Kedushah. The Rav notes that this is in contrast to the ruling of the Tehilah L’David (a”h), who maintained that one may not interrupt his recitation of a berachah achronah even to answer to Kadish or Kedushah. The Tehila L’David claimed that the laws of Hefsek (interruptions) that apply to Birkat Ha’mazon apply as well to the recitation of a Beracha Me’en Shalosh. Therefore, just as one should not interrupt Birkat Ha’mazon to answer to Kadish or Kedushah, similarly, one should not interrupt a Me’en Shalosh for this purpose.
The Tehilah L’David draws proof to his claim from the Shulhan Aruch’s ruling that a Berachah Me’en Shalosh must be recited while seated, just like Birkat Ha’mazon, thus indicating that Me’en Shalosh is subject to the same Halachot as Birkat Ha’mazon. Ha’Rav Ben Tzion, however, dismisses this proof, noting that the Shulhan Aruch’s ruling does not necessarily indicate complete equivalence between the laws of Birkat Ha’mazon and those that apply to Me’en Shalosh. The fact that they both must be recited in a sitting position does not prove that they share the same Halachot concerning Hefsek. Therefore, even though one should not interrupt Birkat Ha’mazon to answer to Kadish or Kedushah, one should interrupt a Beracha Me’en Shalosh for this purpose.
This is the Halachah.