3. Let $\left|a_{n}\right|(n \geqslant 1)$ be an arithmetic sequence with a common difference less than 0, and let the sum of the first $n$ terms be $S_{n}$, then ( ).
A. $n a_{n} \leqslant S_{n} \leqslant n a_{1}$
B. $n a_{1} \leqslant \mathrm{~S}_{n} \leqslant n a_{n}$
C. $S_{n} \geqslant S_{n+1}$
D. $S_{n} \leqslant S_{n+1}$
3. A

Let $\left\{a_{n}\right\}$ have the first term as $a_{1}$, and the common difference as $d(d<0)$.
Then $n a_{n}=n\left(a_{1}+(n-1) d\right)=n a_{1}+n(n-1) d$.
$$
\begin{array}{l}
\mathrm{S}_{n}=a_{1}+a_{2}+\cdots+a_{n}=n a_{1}+(1+2+\cdots+(n-1)) d=n a_{1}+\frac{n(n-1)}{2} d \\
n a_{1}=n a_{1}
\end{array}
$$

Since $d<0$, it follows that $n a_{n} \leqslant S_{n} \leqslant n a_{1}$.