form type da
->add('Il', null, array('label'=>'Il','attr'=>array('class'=>'form-control')))

->add('Il', null, array('label'=>'Il','attr'=>array('class'=>'form-control select2me')))

new twige ekle

<div>
                            <label for="ilcesecmealani">İlçe</label>
                            <select id="ilcesecmealani" class="form-control select2-option-dalyan ilcedoldur">
                                <option selected="selected" value="0" disabled>İl Seçiniz</option>
                            </select>
                        </div>
                        <div>
                            <label for="semtsecmealani">Semt</label>
                            <select id="semtsecmealani" class="form-control select2-option-dalyan semtdoldur">
                                <option selected="selected" value="0" disabled>İlçe Seçiniz</option>
                            </select>
                        </div>
                        <label for="web_adminbundle_rentacar_Mahalle" class="required">Mahalle</label>
                        <select id="web_adminbundle_rentacar_Mahalle" name="mahalle" class="form-control mahalledoldur select2-option-dalyan">
                        </select>
                        <br>






  {% block script %}
      <script>
          $(document).ready(function(){
              $('.ilcedoldur')
                  .empty()
                  .append('<option selected="selected" value="0" disabled>İl Seçiniz</option>');
              $(".ilsecmealani").change(function() {
                  ilcegetir($(this).val(),"");
              });
              $('.semtdoldur')
                  .empty()
                  .append('<option selected="selected" value="0" disabled>İlçe Seçiniz</option>');
              $("#ilcesecmealani").change(function() {
                  semtgetir($(this).val(),"");
              });
              $('.mahalledoldur')
                  .empty()
                  .append('<option selected="selected" value="0" disabled>Semt Seçiniz</option>');
              $("#semtsecmealani").change(function() {
                  mahallegetir($(this).val(),"");
              });
          });
          
      </script>
  {% endblock %}

Kontroller a ekle
 $em = $this->getDoctrine()->getManager(); kodunun altına ekle

            if($request->get('ilce'))
            {
            $ilce = $em->getRepository('WebAdminBundle:Ilce')->find($request->get('ilce'));
            $otel->setIlce($ilce);
            }
            if($request->get('semt'))
            {
                $semt = $em->getRepository('WebAdminBundle:Semt')->find($request->get('semt'));
                $otel->setSemt($semt);
            }
            if($request->get('mahalle')) {
                $mahalle = $em->getRepository('WebAdminBundle:Mahalle')->find($request->get('mahalle'));
                $otel->setMahalle($mahalle);
            }


base.html.twig ekle

<script type="text/javascript">
    $(document).ready(function(){
        $('.inline').click();
        $('#cboxOverlay').click();
    });
    function ilcegetir(idsibu,selected) {
        $.post("{{path('ilce_getir')}}",{'id':idsibu},function(data){
            var datas = $.parseJSON(data);
            $('.ilcedoldur').empty();
            if(datas[0])
            {
                $('.ilcedoldur').append('<option value="0" selected="selected" disabled>İlçeler yüklendi..</option>');
                $.each(datas, function (index, value) {
                    baslik = value.baslik;
                    id     = value.id;
                    select = "";
                    if(id==selected)
                    {
                        select = 'selected="selected"';
                    }
                    $('.ilcedoldur').append('<option value="'+id+'" '+select+'>'+baslik+'</option>');
                });
            }
            else
            {
                $('.ilcedoldur')
                    .append('<option selected="selected" value="0" disabled>Seçilen ile ait ilçe bulunamadı.</option>');
            }
            $(".select2-option-dalyan").select2();
        });
    }
    function semtgetir(idsibu,selected) {
        $.post("{{path('semt_getir')}}",{'id':idsibu},function(data){
            var datas = $.parseJSON(data);
            $('.semtdoldur').empty();
            if(datas[0])
            {
                $('.semtdoldur').append('<option value="0" selected="selected" disabled>Semtler yüklendi..</option>');
                $.each(datas, function (index, value) {
                    baslik = value.baslik;
                    id     = value.id;
                    select = "";
                    if(id==selected)
                    {
                        select = 'selected="selected"';
                    }
                    $('.semtdoldur').append('<option value="'+id+'" '+select+'>'+baslik+'</option>');
                });
            }
            else
            {
                $('.semtdoldur')
                    .append('<option selected="selected" value="0" disabled>Seçilen ilçeye ait semt bulunamadı.</option>');
            }
            $(".select2-option-dalyan").select2();
        });
    }
    function mahallegetir(idsibu,selected) {
        $.post("{{path('mahalle_getir')}}",{'id':idsibu},function(data){
            var datas = $.parseJSON(data);
            $('.mahalledoldur').empty();
            if(datas[0])
            {
                $('.mahalledoldur').append('<option value="0" selected="selected" disabled>Mahalleler yüklendi..</option>');
                $.each(datas, function (index, value){
                    baslik = value.baslik;
                    id     = value.id;
                    select = "";
                    if(id==selected)
                    {
                        select = 'selected="selected"';
                    }
                    $('.mahalledoldur').append('<option value="'+id+'" '+select+'>'+baslik+'</option>');
                });
            }
            else
            {
                $('.mahalledoldur')
                    .append('<option selected="selected" value="0" disabled>Seçilen semte ait mahalle bulunamadı.</option>');
            }
            $(".select2-option-dalyan").select2();
        });
    }
</script>

getir

new.html.twig form bitişinin başına ekle
<div>
                        <label for="ilcesecmealani">İlçe</label>
                        <select id="ilcesecmealani" name="ilce" class="form-control select2me ilcedoldur">
                            <option selected="selected" value="0" disabled>İl Seçiniz</option>
                        </select>
                    </div>
                    <div>
                        <label for="semtsecmealani">Semt</label>
                        <select id="semtsecmealani" name="semt" class="form-control select2me semtdoldur">
                            <option selected="selected" value="0" disabled>İlçe Seçiniz</option>
                        </select>
                    </div>
                    <label for="web_adminbundle_rentacar_Mahalle" class="required">Mahalle</label>
                    <select id="web_adminbundle_rentacar_Mahalle" name="mahalle" class="form-control mahalledoldur select2me">
                    </select>
                    <br>



edit.twig.html ekle

form_widget altına ekle
<div>
                        <label for="ilcesecmealani">İlçe</label>
                        <select id="ilcesecmealani" class="form-control select2-option-dalyan ilcedoldur">
                            <option selected="selected" value="0" disabled>İl Seçiniz</option>
                        </select>
                    </div>
                    <div>
                        <label for="semtsecmealani">Semt</label>
                        <select id="semtsecmealani" class="form-control select2-option-dalyan semtdoldur">
                            <option selected="selected" value="0" disabled>İlçe Seçiniz</option>
                        </select>
                    </div>
                    <label for="web_adminbundle_rentacar_Mahalle" class="required">Mahalle</label>
                    <select id="web_adminbundle_rentacar_Mahalle" name="mahalle" class="form-control mahalledoldur select2-option-dalyan">
                    </select>
                    <br>


Sayfanın en altına
{% block script %}
        <script>
            $(document).ready(function(){
                $('.ilcedoldur')
                    .empty()
                    .append('<option selected="selected" value="0" disabled>İl Seçiniz</option>');
                $(".ilsecmealani").change(function() {
                    ilcegetir($(this).val(),"");
                });
                $('.semtdoldur')
                    .empty()
                    .append('<option selected="selected" value="0" disabled>İlçe Seçiniz</option>');
                $("#ilcesecmealani").change(function() {
                    semtgetir($(this).val(),"");
                });
                $('.mahalledoldur')
                    .empty()
                    .append('<option selected="selected" value="0" disabled>Semt Seçiniz</option>');
                $("#semtsecmealani").change(function() {
                    mahallegetir($(this).val(),"");
                });
                {% if otel.il and otel.ilce %}
                ilcegetir({{ otel.il.id }},{{ otel.ilce.id }});
                {% endif %}
                {% if otel.ilce and otel.semt %}
                semtgetir({{ otel.ilce.id }},{{ otel.semt.id }});
                {% endif %}
                {% if otel.semt and otel.mahalle %}
                mahallegetir({{ otel.semt.id }},{{ otel.mahalle.id }});
                {% endif %}
            });
        </script>
    {% endblock %}