How to search & replace only the first 3 digitsof a 10 digit #? Newbie asks!

[Mike Charles]

What regular expression might I use to replace area codes in a telephone #(or just the first 3 digits of a number. I can't use just a simple search and replace. For example: 4062324064 The new # would change to ex:302233024. I of course only want the area code or the first 3 digits to be searched.   I am brand new to this, yet I a already have a need to replace area codes on a list. Your help is apreciated.   Mike Charles