I tried your suggestion for adding the extra address field to the registration page but obviously it wasn't styled the same as the other fields so I then tried the method I described in my earlier post by copying, pasting and editing the Address2 field code to produce the Address3 field code.
I also used the same method to add Address3 code in the $WA_fieldNamesStr area of the registration page, not sure if this was essential.
I also used the same method to add the extra Addres3 fields to the checkout and confirm pages.
Everything seems to work except for one issue. When the checkbox is selected to copy the billing info to the shipping info on the checkout page the new Address3 info does not copy to the shipping area on both the checkout and confirm pages.
Can you resolve this issue and can you see any unforseen problems with this whole approach.