Function Help? Remove first 3 digits from Phone # depending on Picklist value?

Function Help? Remove first 3 digits from Phone # depending on Picklist value?

In Leads, Standard Layout, I have a Picklist called "Result" with the values of either:
  1. Val1
  2. Val2
  3. Val3
  4. Val4
  5. Val5
I have a custom Phone number field called "Phone3". with a 10-digit phone format. ##########.

We need a function that removes the first 3 digits in the "Phone3" Field when "Result" value is either "Val1" or "Val2".  Can you help us write something that does this? It seems rather complicated.

Example: This...(phone3 value is actually "5025554444" without the () or - 
Would become this automatically when Val1 or Val2 is selected: "5554444"