How to create a deadline column for it to be three days before the deadline that was put in the form

I am trying to create a workflow (I think) that when someone completes my form and enters their requested deadline in the form, there will be an automatic column that will show three days before that deadline requested. I want that date to automatically show three days before the requested deadline so that we can get enough time to have approval from our managers and make any changes to what is being requested before the requested deadline.

Basically, how do I have a column to automatically show three days before a requested deadline?