1 Validator on 3 controls

Hello,
My question is: how can I make a validator supervise on 3 different controls?
In my Register form I have 3 DropDownLists indicating Day,Month and Year.
Their default values are (DD) (MM) (YYYY) and I want to make the validator understand that these values are forbidden for the user to select. Now, I prefer artisticly not to create 3 RequiredFieldValidators, what can I do in that case?
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