Sounds like you need a fifth field.
If ( Not IsEmpty ( Full name ) ; Full Name ; Substitute ( List ( First ; Middle ; Last ) ; ¶ ; " " ) )
Note: this assumes that you want the complete name to be FirstName Middle LastName, with a single space between the names. If you want last name first with a comma after the last name, a different calculation would be needed.
That is exactly what I needed. I already had a fifth field "DisplayName" but I just couldn't figure out what type of formula to use.