27x³ - 27x² y + 9xy²-y³ - 20x³ + 15x² y - 4xy² + 3y³ - 10x³ - 12x² y +5xy² - y³

To simplify the expression, we can group like terms:

(27x³ - 20x³ - 10x³) + (-27x² y + 15x² y - 12x² y) + (9xy² - 4xy² + 5xy²) + (-y³ + 3y³ - y³)

This simplifies to:

(-3x³) + (-24x² y) + (10xy²) + (-y³)

So, the simplified expression is:

-3x³ - 24x² y + 10xy² - y³