First let's analyse the product of the signs.
We have 3 minus signs. The product of two of them will be a positive sign. Then, the product of the resulting positive sigh with the negative sign will be a negative sign.
Now, let's calculate the module of the product.
First let's multiply 5 * 6:
5 * 6 = 30
Now, we multiply the result 30 by the number 3:
30 * 3 = 90
So the result of the product is 90 with a negative sign, that is, -90.