Help with a for .each in jquery

Go through all the elements by a class common to all and ask the id:

$(".numeros").each(function() {
    var id = "#"+this.id;   
    alert(id);
});

You can go through the elements of DOM with .each() of jQuery, to do it first you have to select which elements are going to be selected.

If selected by class, the jQuery selector will return an array with all selected elements $('.clase') .

Now we just have to apply the .each() method to go through the elements, so the function would be as follows:

$('.numeros').each(function(index, elem) {
  //index: Posision del elemento actual de 0 - N.
  //elem: Elemento actual del DOM que esta seleccionado
  console.log($(elem).attr('id'));
});

Example: (This is an example of what could apparently be seen in the browser)

$(function() {
  var arrIds = [];
  $('.numeros').each(function(index, elem) {
    console.log($(elem).attr('id'));
  })

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

<div class="form-group">
  <div class="col-md-12">
    <input class="form-control numeros" type="text" id="3_001" name="3_001">
  </div>
</div>
<div class="form-group">
  <div class="col-md-12">
    <input class="form-control numeros" type="text" id="3_002" name="3_002">
  </div>
</div>
<div class="form-group">
  <div class="col-md-12">
    <input class="form-control numeros" type="text" id="3_003" name="3_003">
  </div>
</div>
<div class="form-group">
  <div class="col-md-12">
    <input class="form-control numeros" type="text" id="3_004" name="3_004">
  </div>
</div>

I have left you a small example so you can learn how it works and execute it.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>
<script>
  function tomarElementos(){
    var respuesta = "";
    var maximoElementos = $(".numeros").size();
    for(i = 0; i<maximoElementos; i++){
      respuesta = respuesta + "/" + $(".numeros:eq("+i+")").val();
    }
    alert(respuesta);
  }
  
</script>
<input class="numeros" type="text" id=""  name="" value="asd0">
<input class="numeros" type="text" id=""  name="" value="asd1">
<input class="numeros" type="text" id=""  name="" value="asd2">
<input class="numeros" type="text" id=""  name="" value="asd3">
<button onclick="tomarElementos()">clickme</button>

In your particular case it is as follows:

<tr>
<td>Número de estudiantes del turno según SIAGIE</td>
<?php foreach ($niveles as $obj) { ?>
<td>
<div class="form-group">
<div class="col-md-12">
<input class="form-control numeros" type="text" id="3_<?php echo str_replace(' ', '_', $obj->nivel) . '_' . $obj->codigoModular; ?>"  name="3_<?php echo str_replace(' ', '_', $obj->nivel) . '_' . $obj->codigoModular; ?>">
</div>
</div>
</td>
<?php } ?>

Jquery

<script>
    var respuesta = ""; //Variable para salida
    var maximoElementos = $(".numeros").size(); //Define la cantidad de nodos con esta clase
    for(i = 0; i<maximoElementos; i++){ //recorre la n cantidad de nodos
      respuesta = respuesta + "/" + $(".numeros:eq("+i+")")attr("id"); //va asignando a respuesta, el dato anterior mas el valor del nuevo nodo
    }

    alert(respuesta); //Lanza el resultado del recorrido
</script>

I hope you can understand and be of help. Anything please let me know in the comments to guide you.

Success in your project!