If is [tex]\dfrac{3^3}{8}[/tex] then your answer is
[tex]\dfrac{3\cdot3\cdot3}{8}=\dfrac{27}{8}[/tex]
If is [tex]\left(\dfrac{3}{8}\right)^3[/tex] then your answer is
[tex]\dfrac{3}{8}\cdot\dfrac{3}{8}\cdot\dfrac{3}{8}=\dfrac{27}{512}[/tex]
Other method.
Use [tex]\left(\dfrac{a}{b}\right)^n=\dfrac{a^n}{b^n}[/tex]
[tex]\left(\dfrac{3}{8}\right)^3=\dfrac{3^3}{8^3}=\dfrac{27}{512}[/tex]