The answer is $\left(g \setminus \circ h\right) \left(3\right) = 24$.
By definition, $\left(g \setminus \circ h\right) \left(3\right) = g \left(h \left(3\right)\right)$ (plug the output of the inner function into the outer function).
Since $h \left(3\right) = {3}^{2} - 2 = 9 - 2 = 7$ and $g \left(7\right) = 3 \cdot 7 + 3 = 21 + 3 = 24$, it follows that $\left(g \setminus \circ h\right) \left(3\right) = 24$.