Ask yourself: how many eighths in 1? The answer is in the question: eight. 8 eighths make up one whole or the whole of 1. In your question you have 3 eighths, so how many more will you need to make 8 eighths? If you had 3 apples how many more apples would you need to make 8 apples? The answer for the apples would be to take away 3 apples from 8 leaving 5. It's the same with eighths: you need 5 more eighths to make 8 eighths, or 1.
3 eighths is written 3/8, so 5 eighths is written 5/8. That's your answer!