\(f\left(3\right)+2f\left(\frac{1}{3}\right)=2.3=6\)=>\(2f\left(\frac{1}{3}\right)=6-f\left(3\right)\Leftrightarrow f\left(\frac{1}{3}\right)=3-\frac{f\left(3\right)}{2}\)
\(f\left(\frac{1}{3}\right)+2f\left(3\right)=2.\frac{1}{3}=\frac{2}{3}\)=> \(3-\frac{f\left(3\right)}{2}+2f\left(3\right)=\frac{2}{3}\Leftrightarrow\frac{3}{2}f\left(3\right)=\frac{2}{3}-3=-\frac{7}{3}\)
\(f\left(3\right)=-\frac{14}{9}\)