Case 1 of master theorem as
$f(n) = O\left(n^{\log_b a -\epsilon} \right ) \\= O\left(n^{\log_2 8 -\epsilon }\right) \\= O\left(n^{3 -\epsilon} \right)$
is true for any $ 0 < \epsilon \leq 2$.
Now, the complexity is $\Theta\left(n^{ \log_b a }\right) = \Theta \left(n^3\right)$.