To find \( f^{-1}(5) \), we start by interpreting the meaning of the notation \( f^{-1}(y) \). Specifically, \( f^{-1}(y) \) gives us the value of \( x \) such that \( f(x) = y \).
Given that \( f(3) = 5 \), we can conclude that:
\[ f^{-1}(5) = 3 \]
Thus, the answer is:
\[ \boxed{3} \]