3. Let $x \in\left(-\frac{3 \pi}{4}, \frac{\pi}{4}\right)$, and $\cos \left(\frac{\pi}{4}-x\right)=-\frac{3}{5}$. Then the value of $\cos 2 x$ is ( ).
(A) $-\frac{7}{25}$
(B) $-\frac{24}{25}$
(C) $\frac{24}{25}$
(D) $\frac{7}{25}$
3. B.

From the given, $\sin \left(\frac{\pi}{4}-x\right)=\frac{4}{5}$. Therefore, $\cos 2 x=2 \sin \left(\frac{\pi}{4}-x\right) \cdot \cos \left(\frac{\pi}{4}-x\right)=-\frac{24}{25}$.