3. There are six numbers $a b, a(a+6), a(b+6), b(a+6)$, $b(b+6),(a+6)(b+6)$ for any pair of natural numbers $a$ and $b, a \neq b$. Among them, for some $a$ and $b$, perfect squares can be found. For which $a$ and $b$ will the number of perfect squares be maximized?
Answer: three squares when $a=2, b=2 k^{2}, k \in Z, k>1$.