\(f\left(x\right)-3f\left(\frac{1}{x}\right)=x^2\Rightarrow f\left(\frac{1}{x}\right)-3f\left(x\right)=\frac{1}{x^2}\)
\(\Leftrightarrow3f\left(\frac{1}{x}\right)-9f\left(x\right)=\frac{3}{x^2}\)
\(\Rightarrow-8f\left(x\right)=x^2+\frac{3}{x^2}\Rightarrow f\left(x\right)=-\frac{1}{8}x^2-\frac{3}{8x^2}\)