The triple angle formula states that $ \cos 3 \theta = 4 \cos^3 \theta - 3 \cos \theta $. Then \[\cos^3 \theta = \frac{1}{4} \cos 3 \theta + \frac{3}{4} \cos \theta.\]Hence, \begin{align*} \cos^3 \frac{2 \pi}{7} + \cos^3 \frac{4 \pi}{7} + \cos^3 \frac{8 \pi}{7} &= \left( \frac{1}{4} \cos \frac{6 \pi}{7} + \frac{3}{4} \cos \frac{2 \pi}{7} \right) + \left( \frac{1}{4} \cos \frac{12 \pi}{7} + \frac{3}{4} \cos \frac{4 \pi}{7} \right) + \left( \frac{1}{4} \cos \frac{24 \pi}{7} + \frac{3}{4} \cos \frac{8 \pi}{7} \right) \\ &= \frac{1}{4} \left( \cos \frac{6 \pi}{7} + \cos \frac{12 \pi}{7} + \cos \frac{24 \pi}{7} \right) + \frac{3}{4} \left( \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{8 \pi}{7} \right) \\ &= \frac{1}{4} \left( \cos \frac{6 \pi}{7} + \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} \right) + \frac{3}{4} \left( \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{6 \pi}{7} \right) \\ &= \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{6 \pi}{7}.\end{align*}Consider the sum \[S = \operatorname{cis} 0 + \operatorname{cis} \frac{2 \pi}{7} + \operatorname{cis} \frac{4 \pi}{7} + \dots + \operatorname{cis} \frac{12 \pi}{7}.\]Then \begin{align*} S \operatorname{cis} \frac{2 \pi}{7} &= \operatorname{cis} \frac{2 \pi}{7} + \operatorname{cis} \frac{4 \pi}{7} + \dots + \operatorname{cis} \frac{12 \pi}{7} + \operatorname{cis} 2 \pi \\ &= \operatorname{cis} \frac{2 \pi}{7} + \operatorname{cis} \frac{4 \pi}{7} + \dots + \operatorname{cis} \frac{12 \pi}{7} + \operatorname{cis} 0 \\ &= S, \end{align*}so $ S \left( 1 - \operatorname{cis} \frac{2 \pi}{7} \right) = 0 $. Hence, $ S = 0 $. Taking the real part of $ S $ gives us \[\cos 0 + \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{6 \pi}{7} + \cos \frac{8 \pi}{7} + \cos \frac{10 \pi}{7} + \cos \frac{12 \pi}{7} = 0.\]Then \[1 + \cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{6 \pi}{7} + \cos \frac{6 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{2 \pi}{7} = 0,\]so \[\cos \frac{2 \pi}{7} + \cos \frac{4 \pi}{7} + \cos \frac{6 \pi}{7} = \boxed{-\frac{1}{2}}.\]