Enable Disable RequiredField Validators based on Value selected in DropDownList in ASP.Net

I have a dropdownlist ,three text boxes and a submit button.The three text boxes are disabled and enabled based on the condition in dropdownlist.I have provided required field validators in three text boxes.When I click on condition "self" then three text boxees are disabled and submit button click wont enter the values since required field validators prevent that.I have set causes validation property to false.But it wont work for the second condition where i need to enable all the textboxes,If i didnt provide any values it will enter nulll values to db.How should I resolve this???/