3. Given the three sides of $\triangle A B C$ are $a, b, c$. If $\frac{1}{a}, \frac{1}{b}, \frac{1}{c}$ form an arithmetic sequence in order, then $\angle B($ ).
(A) must be an acute angle
(B) must be a right angle
(C) must be an obtuse angle
(1)) cannot be determined, depends on the values of $a, b, c$
3. (A).

From $\frac{1}{a} 、 \frac{1}{b} 、 \frac{1}{c}$ forming an arithmetic sequence, we know that
$$
\frac{1}{a} \geqslant \frac{1}{b} \geqslant \frac{1}{c} \text { or } \frac{1}{a} \leqslant \frac{1}{b} \leqslant \frac{1}{c} \text {, }
$$

which means $a \leqslant b \leqslant c$ or $a \geqslant b \geqslant c$.
By the principle that the larger side faces the larger angle, we always have $\angle B$ between $\angle A$ and $\angle C$. Furthermore, since a triangle must have at least two acute angles, $\angle B$ must be an acute angle.