According to the parity rule of integer addition:

Odd+odd = even

Odd+even = odd

Even number+even number = even number

In this question, all the numbers that can be filled in the three brackets are odd. According to the parity law of integer addition, the final result can only be odd, not even.

30 is an even number, so no matter how you fill it in, it is impossible to get 30 by adding three odd numbers.