3. If $n \in \mathbf{R}$, and the equation $\sqrt{x^{2}-9}=-x-n$ has one solution, then the range of values for $n$ is $\qquad$ .
3. $n \leqslant-3$ or $0<n \leqslant 3$.
$$
\text { Let } y=\sqrt{x^{2}-9} \text {. }
$$

Then $y \geqslant 0 . x^{2}-y^{2}=9$ is the part of the hyperbola above and on the $x$-axis (as shown in Figure 3).
$$
\text { Let } y=-x-n \text {. }
$$

This is a line.
From the original, we find one
point $1(-3,0)$ to
$t_{1}$ intersection
$$
y=-x-3 \text {. }
$$
$$
\begin{array}{l}
(3,+\infty) \text { is small. Therefore, } \\
0<n \leqslant 3 \text { and } n \leqslant-3 .
\end{array}
$$