17. (12 points) Given the function $f(x)$ for any real numbers $x, y$, it satisfies $f(x+y)=f(x)+f(y)-3$, and when $x>0$, $f(x)<3$.
(1) Is $f(x)$ a monotonic function on the set of real numbers $\mathbf{R}$? Explain your reasoning;
(2) If $f(6)=-9$, find $f\left(\left(\frac{1}{2}\right)^{2010}\right)$.