If a, b and c are three rational numbers, we have:

1. (a x b) x c = (a x c) x (b x c)
2. (a + b) x c = (a + c) x (b + c)
3. a x (b - c) = a x b - a x c
4. a x (b - c) = a x b - c

We know that, multiplication of rational numbers is distributive over their addition/subtraction.

∴ a x (b - c) = a x b - a x c

Hence, option 3 is the correct option.