validation checks

Hi guys,
 I hope this makes sense...
I have a form where the user can enter the details of 3 of their pets. I have the required field validation set for the firstname, surname and email address for all three but would like to allow the user to enter only one if thats all they want to enter.
Therefore I want the validation checks to be disabled unless the user has entered something into the textbox. ie if they enter name and surname then i want a check to make sure they have entered email address etc.
 
Thanks

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