Is it true that the smallest root $t$ of the polynomial

$$ 20 t^3 - 30 t^2 + (12 - 4 \cos^2 \alpha - 4 \cos^2 \beta - 4 \cos^2 \gamma) t + \cos^2 \alpha + \cos^2 \beta + \cos^2 \gamma - 2 \cos \alpha \cos \beta \cos \gamma - 1 $$

is always no greater than $\frac{2}{5}$ for all $\alpha, \beta, \gamma \in [0,\pi]$ with $\alpha + \beta + \gamma \leq 2\pi$?

Thanks a lot for any helpful answer.