Vector Algebra

Let $\bar{a}, \bar{b}, \bar{c}$ be three vectors of equal magnitude such that the angle between $\bar{a}$ and $\bar{b}$ is $\alpha$, $\bar{b}$ and $\bar{c}$ is $\beta$, $\bar{c}$ and $\bar{a}$ is $\gamma$.
Then the minimum value of $\cos\alpha + \cos\beta + \cos\gamma$ is ...