create dropdown list from text fields

I am using Acrobat Pro DC.  I have a form that has multiple text fields.  I want to create a dropdown that populates from the values of those text fields.

I have fields:

     Goal1

     Goal2

     Goal3

The user enters the text for each goal.  At the end of the form, they get a drop down field, "Priority", with the data they entered from Goal1, Goal2, Goal3.

Should be easy, but I cannot figure this out.