Let \( |\cos \theta\cos(60-\theta)\cos(60+\theta)\le \frac{1}{8},\ \theta\in[0,2\pi] \). Then, the sum of all \( \theta\in[0,2\pi] \), where \( \cos 3\theta \) attains its maximum value, is:
\( 15\pi \)
\( 9\pi \)
\( 6\pi \)
\( 18\pi \)