Step 1 :Let $f(x)=\sqrt{x}$
Step 2 :Find $g(x)$, the function that is $f(x)$ reflected over the $x$-axis and horizontally stretched by a factor of 3
Step 3 :The reflection of a function over the x-axis is achieved by taking the negative of the function
Step 4 :The horizontal stretch by a factor of 3 is achieved by replacing x with x/3 in the function
Step 5 :Apply these transformations to the function $f(x) = \sqrt{x}$
Step 6 :So, $g(x) = -\sqrt{x/3}$
Step 7 :Simplify the final answer
Step 8 :\(\boxed{g(x) = -\frac{\sqrt{3}}{3}\sqrt{x}}\)