Answering Kedusha While in Shemoneh Esrei
Question
If one said yehiyu l’ratzon after sim shalom, but hasn’t taken three steps back, how does one respond to kedusha?
Answer
Shalom!
Thank you for your question.
Once a person has said “yihiyu l’ratzon...” one is permitted to answer “Kadosh…” and “Baruch kevod…” in Kedusha along with everyone else.
The reason why this is allowed is because once one has recited “yihiyu l’ratzon…” one is subject to the same halacha as if one is in the middle of the blessings of the Shema. And during the blessings of the Shema these responses are permitted, as well.
Allow me to add that once one has said the second “yihiyu l’ratzon…” (which is a near universal practice), one is permitted to answer “amen” to any blessing and to essentially answer any congregational response whatsoever.
Source
OC 122:1, Mishna Berura 122:1-4