ĐK: \(\left\{ \begin{array}{l}\cos x \ne 0\\\cos 3x \ne 0\end{array} \right.\)\( \Leftrightarrow \left\{ \begin{array}{l}\cos x \ne 0\\4{\cos ^3}x - 3\cos x \ne 0\end{array} \right.\)\(  \Leftrightarrow \left\{ \begin{array}{l}\cos x \ne 0\\4{\cos ^2}x - 3 \ne 0\end{array} \right. \)\( \Leftrightarrow \left\{ \begin{array}{l}\cos x \ne 0\\\cos x \ne \pm \frac{{\sqrt 3 }}{2}\end{array} \right. \)\( \Leftrightarrow \left\{ \begin{array}{l}x \ne \frac{\pi }{2} + k\pi \\x \ne \pm \frac{\pi }{6} + k2\pi \end{array} \right.\)

\(\tan x = \tan 3x \)\( \Leftrightarrow 3x = x + k\pi \)\( \Leftrightarrow 2x = k\pi\)\(  \Leftrightarrow x = \frac{{k\pi }}{2}\,\,\left( {k \in Z} \right)\).

Đối chiếu điều kiện ta có \(x = k\pi \,\,\left( {k \in Z} \right)\).

Chọn B.