Optional and Mandatory field in the combination

Hi SAP Experts,

I want to know which User exit needs to be modified for below requirement.

XD01> General data level

A.Tax Number 1
B. Tax Number 2
C.VAT Reg No

One of three field should be mandatory...

Example : Suppose user not maintained A,B but C should be mandatory field.

                If User  maintained Tax number then B,C would be optional....Any one of above fields should be entered.

Thanks

Venugopal