Let \( f:\mathbb{R}\to \mathbb{R} \) be a differentiable function such that \( f\!\left(\frac{x+y}{3}\right)=\frac{f(x)+f(y)}{3} \) for all \( x,y\in \mathbb{R} \), and \( f'(0)=3 \). Then the minimum value of the function \( g(x)=3+e^{x}f(x) \), is:
