$$(\frac{1}{x}+\frac{y}{3})^{3}$$
$\frac{\left(y+\frac{3}{x}\right)^{3}}{27}$
$\frac{\left(xy\right)^{2}}{3x^{3}}+\frac{y^{3}}{27}+\frac{y}{x^{2}}+\frac{1}{x^{3}}$
