Thank you for such a quick response!
That did not work.
First I copied and pasted your code as you sent it, it did not work. I ended up with the following:
Max([Referral Date],[2nd Referral Date],[3rd Referral Date])
If(
Max([referral date],[2nd referral date],[3rd referral date])<[most recent referral date],null,{else?})
It keeps giving me the following error: "There are extra characters beyond the end of the formula"