3. Given $a^{-}+a^{-x}=2$. Then the value of $a^{2 x}+a^{-2 x}$ is ( ).
(A) 4
(B) 3
(C) 2
(D) 6
3. (C).
$$
\begin{array}{l}
\because a^{2 x}+a^{-2 x}=\left(a^{x}\right)^{2}+\left(a^{-x}\right)^{2} \\
=\left(a^{x}+a^{-x}\right)^{2}-2=4-2=2 .
\end{array}
$$