Step 1 :
After factoring out $ 3 $ we have:
$$ 3x^{3}-12x^{2}+6x-24 = 3 ( x^{3}-4x^{2}+2x-8 ) $$Step 2 :
To factor $ x^{3}-4x^{2}+2x-8 $ we can use factoring by grouping:
Group $ \color{blue}{ x^{3} }$ with $ \color{blue}{ -4x^{2} }$ and $ \color{red}{ 2x }$ with $ \color{red}{ -8 }$ then factor each group.
$$ \begin{aligned} x^{3}-4x^{2}+2x-8 = ( \color{blue}{ x^{3}-4x^{2} } ) + ( \color{red}{ 2x-8 }) &= \\ &= \color{blue}{ x^{2}( x-4 )} + \color{red}{ 2( x-4 ) } = \\ &= (x^{2}+2)(x-4) \end{aligned} $$