how to use immediate attribute in data table command button?

Question

I tried to create account records using datable.Its worked fine when the command button is .But using immediate button in the command button (i.e)its not working

VF

<apex:page controller="accountcontroller">
  <apex:form >
    <apex:pageBlock >
      <apex:pageBlockSection >
        <apex:dataTable value="{!acc}"   var="a" cellpadding="4" border="1" rows="3" >
          <apex:column headervalue="account name" width="20" >
            <apex:inputField value="{!a.name}" required="true"/> 
          </apex:column>
          <apex:column headervalue="account site" width="20" >
            <apex:inputField value="{!a.site}" required="true"/>
          </apex:column>
          <apex:column headervalue="account phone" width="20" >
            <apex:inputField value="{!a.phone}" required="true"/> 
          </apex:column>
        </apex:dataTable> 
      </apex:pageBlockSection>
      <apex:pageblockButtons >
        <apex:commandButton value="save" action="{!insertNewItem}" immediate="true" />
      </apex:pageblockButtons>
    </apex:pageBlock>
  </apex:form>
</apex:page>

apex

public class accountcontroller {
    public list<account> acc{get;set;}

    public accountcontroller(){
        acc= new list<account>();
        for ( integer i = 0 ; i < 3 ;i++ ) {
            account ac = new account();
            acc.add(ac);
        }
    }

    public PageReference insertNewItem() {
        insert acc;
        system.debug('<<<<<<<<<<<<'+acc);
        acc= new list<account>();
        /*
        for(integer i=0 ; i<3 ;i++){
            account ac = new account();
            acc.add(ac);}
        */
        return null;
    }

}

Answer 1

I think you want to insert the list of accounts after pressing the command button. so, may be problem here is you are not giving the values to the required fields of an account, without giving values to the required field new account can not be inserted as immediate attribute only bypasses the validation rules.