For this case we have to find out how much younger sister I was three years ago.
Let the variable "x" be my age. So:
If my brother is 5 years older then:

If my sister is 3 years younger then:

For any value of "x", that is, my age, my brother will always be 5 years older and my sister 3 years younger. Then, three years ago when I was x years old, my sister was three years younger than me.
Answer:
she was three years younger than me