3. The monotonic increasing interval of the function $f(x)$ $=\frac{(x+1)^{2}}{x^{2}+1}$ is $\qquad$
$$
f(x)=1+\frac{2 x}{x^{2}+1}, f^{\prime}(x)=\frac{2\left(x^{2}+1\right)-2 x \cdot 2 x}{\left(x^{2}+1\right)^{2}}=-\frac{2(x+1)(x-1)}{\left(x^{2}+1\right)^{2}}>0
$$
$\Rightarrow -1<x<1$, so the monotonic increasing interval of $f(x)$ is $(-1,1)$.