3.计算下列极限.
(1) $\lim_{x \rightarrow + \infty }( \sqrt {x^{2}+4x+1}- \sqrt {x^{2}-2x+3})$.
(2) $\lim_{x \rightarrow + \infty }(x \arctan x- \dfrac { \pi }{2}x)$.
(3) $\lim_{x \rightarrow 0} \left ( \frac {1}{x}- \dfrac {1}{e^{x}-1} \right )$.
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$