3. Given the set of real numbers as $\mathrm{R}$, the function $f(x)=x^{2}+a x+b(a, b \in \mathrm{R})$, and $A=$ $\{x \mid x=f(x), x \in \mathrm{R}\}, B=\{x \mid x=f(f(x)), x \in \mathrm{R}\}$, then (1) when $A=$ $\{-1,3\}$, $B=$ $\qquad$ ;(2) the relationship between $A$ and $B$ $\qquad$ .
$\{-\sqrt{3},-1, \sqrt{3}, 3\}, A \subseteq B;$