Strange Cross Products

Geometry Level 1

Let $\vec{a},\vec{b}$ and $\vec{c}$ be three non-zero vectors such that no two of them are collinear and $(\vec{a} \times \vec{b}) \times \vec{c}=\frac{1}{3} |\vec{b}||\vec{c}| \vec{a}$ . If $\theta$ is the angle between vectors $\vec{b}$ and $\vec{c}$ , then the value of $\sin\theta$ is :