Given that $\theta$ is a small angle, find an approximation for $\cos (3\theta )$.

Select the correct answer from the options below:

$1-\frac{9{\theta }^2}{2}$

$1-\frac{3{\theta }^2}{2}$

$3-\frac{3{\theta }^2}{2}$

$1-9{\theta }^2$