3. Given the equation $\arccos \frac{4}{5}-\arccos \left(-\frac{4}{5}\right)=\arcsin x$, then
A. $x=\frac{24}{25}$
B. $x=-\frac{24}{25}$
C. $x=0$
D. Such an $x$ does not exist
$$
\begin{array}{l}
\arccos \frac{4}{5}-\arccos \left(-\frac{4}{5}\right)=\pi-2 \arccos \frac{4}{5}, \\
\text { Since } \frac{4}{5}>\frac{\sqrt{2}}{2} \Rightarrow \arccos \frac{4}{5}<\frac{\pi}{2},
\end{array}
$$

Therefore, such an $x$ does not exist. So the answer is $D$.