The displacement of simple harmonic oscillator after \( 3 \, \text{seconds} \) starting from its mean position is equal to half of its amplitude. The time period of harmonic motion is:
\( 6 \, \text{s} \)
\( 8 \, \text{s} \)
\( 12 \, \text{s} \)
\( 36 \, \text{s} \)