is the Derivative of tan3x = tan3?
If it's $\displaystyle \displaystyle \begin{align*} \left( \tan{3} \right) x \end{align*}$, then you are correct. More likely though, it's $\displaystyle \displaystyle \begin{align*} \tan{(3x)} \end{align*}$ which has derivative $\displaystyle \displaystyle \begin{align*} 3\sec^2{(3x)} \end{align*}$.