Multifield validation with variable number of components

Question

I'm building a form in primefaces and need to run validation on a specific field of all items in a <p:dataList>. Specifically I need to make sure there are only a maximum 3 different values in any number of items in the list.

<p:dataList value="#{myBean.myItems}" var="it"
    id="myDataList" rowIndexVar="rowIndex">
   ...
  <p:inputNumber value="#{it.fieldToValidate}">
    <f:validator validatorId="myValidator" />
  </p:inputNumber>
   ...
</p:dataList>

And the validator:

@FacesValidator(value = "myValidator")
public class MyValidator implements Validator {

  @Override
  public void validate(FacesContext context, UIComponent component, Object value)
      throws ValidatorException {

    if (component == null)
      return;
    Set<BigDecimal> myValues = new HashSet<BigDecimal>();

    //now add all values to the set, but how to get them?

    if (myValues.size() > 3) {
      throw new ValidatorException(new FacesMessage(
          FacesMessage.SEVERITY_ERROR, "To many different values",
          "There can be only three"));
    }
  }
}

I have read Validation across multiple fields in JSF/PrimeFaces and

JSF doesn't support cross-field validation, is there a workaround? and

How validate two password fields by ajax?

but they don't solve my problem because I don't know the contents of my data list beforehand.

I am using JSF-2.1.7, Primefaces-6.0.4 and jdk-1.6, ancient I know but it can't be helped...