So for your first project I would do something simple in concept so you don't get wrapped up in feature creep.

Think about what you have learned so far and how you could demonstrate those topics. I don't think it needs to be a fresh new idea.

3 days is pretty short, without knowing what topics you have covered I would pick 3 and think of a way to combine them.

Say you learned a bit of UI, a bit of sorting, and a bit of OOP. Maybe create a todo list app that sorts tasks by priority.