Number Theory

Let $f(n)$ return the number of distinct ordered pairs of positive integers $(a,b)$  such that for each ordered pair, $a^2+b^2=n$ . Note that when $a \neq b, (a,b)$,  and $(b,a)$ are distinct. What is the smallest positive integer $n$ for which $f(n)=3$ ?

Please write an approach to get to the answer.