'Fill up', 'Fill out', 'Fill in'

1. 'Fill up', 'fill out' and 'fill in' are phrasal verbs. All three use the word 'fill', which means 'put something in an empty space'.
2. 'Fill up' means 'make something become full':
1. I filled the car up with fuel
2. There's plenty of food. Fill yourself up.
3. 'Fill out' means 'complete a form or document', usually where spaces have been provided for someone to write in:
1. To join the gym, please fill out this form
2. Fill out the blanks to complete the exercise
4. 'Fill in' can mean...
   ...'make something full' (similar to 'fill up'):
1. The workers filled in a hole in the road 
5. ...'complete a form or document' (like 'fill out'):
1. To join the gym, please fill in this form
6. ...'temporarily replace someone at work':
1. Jane is sick today. Could you please fill in for her?
7. ...'to give someone information about something they don't know':
1. Let me fill you in on what you missed while you were sick.