Let $g(x)$ be a linear function and $ f(x)= \begin{cases} g(x), & x\le 0,\\[2mm] \left(\dfrac{1+x}{2+x}\right)^{\tfrac{1}{x}}, & x>0 \end{cases} $ is continuous at $x=0$. If $f'(1)=f(-1)$, then the value $g(3)$ is