Answer
See the explanation.
Work Step by Step
Let $F=f\circ g\circ h$.
Let $u=h(x)$ and $v=g(h(x))$.
We have $v=g(u)$ and $F=f(v)$.
Using the Chain Rule,
$F'(x)=\frac{dF}{dx}$
$=\frac{dF}{dv}\cdot \frac{dv}{du}\cdot \frac{du}{dx}$
$=f'(v)\cdot g'(u)\cdot h'(x)$
$=f'(g(h(x))\cdot g'(h(x))\cdot h'(x)$
