3. Find all positive integers $n$, such that $2^{n} \mid \left(3^{n}-1\right)$.
$$
\begin{array}{l}
n \leqslant v_{2}\left(3^{n}-1\right) \leqslant 3+v_{2}(n) \\
\Rightarrow n \leqslant 4 .
\end{array}
$$