3. Given the function $f(x)=\cos x \sin 2 x$, which of the following conclusions is incorrect?
A. The graph of $f(x)$ has a center of symmetry
B. The graph of $f(x)$ has an axis of symmetry
C. The equation $f(x)=\frac{7}{9}$ has a solution
D. The equation $f(x)=a$ has an even number of solutions in $[0, \pi)$
$f(x)$ is an odd function, so A is correct; $f\left(\frac{\pi}{2}-x\right)=\sin x \sin 2 x, f\left(\frac{\pi}{2}+x\right)=(-\sin x)(-\sin 2 x)$ $=f\left(\frac{\pi}{2}-x\right) \Rightarrow$ the graph of $f(x)$ is symmetric about the line $x=\frac{\pi}{2}$, so B, D are correct;
$$
f(x)=2 \sin x \cos ^{2} x \Rightarrow|f(x)|=\sqrt{4 \sin ^{2} x \cos ^{2} x \cos ^{2} x} \leqslant \sqrt{2 \cdot\left(\frac{2}{3}\right)^{3}}=\frac{4 \sqrt{3}}{9}<\frac{7}{9} \text {, }
$$

Therefore, the answer is C.