Final answer:
The provided details do not contain information about Sergio and his mom's living situation when he was three years old, and thus, no specific answer can be provided.
Step-by-step explanation:
The information provided is insufficient to answer the question about where Sergio and his mom were living when he was three years old. None of the provided references mention Sergio and his mom's living situation when he was at age three. More context or detail about Sergio is required to determine the correct answer to this question.