The common denominator for each of these terms is x^3 so the new expression would be:
[tex] \frac{x^{3}y}{x^{3}} + \frac{y^{3}}{x^{3}} - \frac{x^{3}y^{3}}{x^{3}} [/tex]
so to simplify that down to a single fraction
[tex] \frac{x^3y + y^{3}-x^{3}y^{3}}{x^{3}} [/tex]
