Answer
$$ y'= \frac{3(\ln (\ln x))^2}{x\ln x}.$$
Work Step by Step
Recall that $(\ln x)'=\dfrac{1}{x}$
Since $ y=(\ln (\ln x))^3$, then by the chain rule, we have
$$ y'=3(\ln (\ln x))^2\frac{1/x}{\ln x}=\frac{3(\ln (\ln x))^2}{x\ln x}.$$