Populating information into another field

Hi! I am creating a form to collect contact information for families of students who go to a charter school. 

https://form.jotform.com/91367243477161

There are 3 sections. Parents may want to fill out all the sections, or just one or two of them.  I was wondering this ... Is there a way to auto-populate a field if the same field has already been filled out? For example, I will have phone number as a field in each section. That way, if someone only wants to fill out that section, we will have the number. But if someone fills out all three sections, they will have to put their number in, which is a pain. What is the best way to avoid that?

Thank you!

Amberly

That was supposed to say "they will have to put their number in 3 times, which will be a pain".

We can create a condition to automatically populate the other fields if the user just wants to update his submission. We will check this question. If the user is updating the information, the form will automatically populate other fields. I have added test fields to show that. 
1. These are my test fields. 2. I created these conditions to copy values to the next field. Please inspect that carefully.  The conditions must seem like that. 
  

On the other hand, I think this article may help you. 
Guide: How to Let Users Update Their Form Submissions at a Later Date