hello ! Fill something in > to write all the necessary information on an official document or form. Besides, you can use fill out in same way.However fill up is a little bit different. If something fills up, or you fill it up, it becomes full. For example : Did you fill the car up ( with petrol ) ? All the best !

In my experience, these phrases are used in the following ways: 1. "fill in" the blank, your name, etc. (used with forms, more specific) 2. "fill in" the form, etc. (used with forms, more general) 3. "fill out" the form, etc. (used with forms, more general) 4. "fill up" the glass with water, etc. (used with liquids, quantity of items, etc.) As you can see, "fill in" and "fill out" are basically the same. "Fill up" is different.