3. Given $\sqrt{a^{2} b}=-a \sqrt{b}$. For the range of values of $a$ and $b$, the correct statement is ( ).
(A) $a \geqslant 0, b \geqslant 0$
(B) $a \leqslant 0, b \geqslant 0$
(C) $a$ is any real number, $b \geqslant 0$
(D) None of the above
3. D.

From $\sqrt{b}$ we know $b \geqslant 0$.
(1) When $b>0$, $\sqrt{a^{2}}=-a \Rightarrow a \leqslant 0$;
(2) When $b=0$, the original equation always holds, $a$ is any real number.

In summary, the range of values for $a$ and $b$ is $a \leqslant 0, b>0$ or $a$ is any real number, $b=0$.