title: Zend Framework 2 - Form Validation - Validator which checks if one of three form fields is given
body: <p>In ZF2 I wrote a custom validator which checks if one of three form elements (element-A, elment-B, elment-C) is given. The custom validator is working fine. A am attching the validator to the form field "element-A".</p>
  
  <p>The issue is: I have to set "setRequired" to "true" for "element-A", otherwise my custom validator is not triggerd. This forces the user to provide a vlaue for the form element the validator is attached to.</p>
  
  <p>How can I trigger my custom validator without this former described behaviour?</p>