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

To simplify the given expression, we can group like terms together and combine them.

Grouping the terms with the same variables, we have:

(27x³ - 20x³ - 10x³) + (-27x²y + 15x²y - 12x²y) + (9xy² - 4xy² + 5xy²) + (-y³ + 3y³ - y³)

Now, let's simplify each group of terms:

(27x³ - 20x³ - 10x³) = -3x³ (-27x²y + 15x²y - 12x²y) = -24x²y (9xy² - 4xy² + 5xy²) = 10xy² (-y³ + 3y³ - y³) = y³

Combining these simplified groups, we get:

-3x³ - 24x²y + 10xy² + y³

So, the simplified form of the expression is:

-3x³ - 24x²y + 10xy² + y³