$\begin{aligned}
& \beta=\frac{\alpha}{2}, \gamma=\frac{\alpha}{2} \
& \cos ^2 \alpha+\cos ^2 \beta+\cos ^2 \gamma=1 \
& \cos ^2 \alpha+2 \cos ^2 \frac{\alpha}{2}=1 \
& \cos ^2 \alpha+\cos \alpha=0 \
& \cos \alpha(\cos \alpha+1)=0 \
& \cos \alpha=0,-1 \
& \alpha=\frac{\pi}{2}, \pi
\end{aligned}$
Now β=2α⇒4π,2π
so sum is 43π