laravel dependent dropdown check

Hi

Have dependent dropdown which return data.

ajax have three values and when all of them is set it gives result.

But I need add check, when a, b , c all this value is set send ajax request , now it send if one of them is set.

I need do this check, in jQuery on in controller.

In controller I try check with $a_id != null && $b_id != null && $c_id != null but this not works. In jQuery I use if(c_id) , but this always give true, also when nothing selected and by default it have Choose

<select id="a_id">
<option selected="selected" value="">Choose</option> 
<option value="list1">list1</option> 
<option value="list2">list2</option> 
<option value="list3">list3</option> 
</select>


<select id="b_id">
<option selected="selected" value="">Choose</option> 
<option value="list22">list2-1</option> 
<option value="list222">list2-2</option> 
<option value="list2222">list2-3</option> 
</select>


<select id="c_id">
<option selected="selected" value="">Choose</option> 
<option value="list33">list3-1</option> 
<option value="list333">list3-2</option> 
<option value="list333">list3-3</option> 
</select>

        $('#a, #b, #c').on('change', function(){ 
            var a_id = $("#a_id").val();
            var b_id = $("#b_id").val();
            var c_id = $("#c_id").val(); 
            if(c_id) {
                $.ajax({
                    url: "{{  url('/get/') }}/"+a_id+"/"+b_id+"/"+c_id,
                    type:"POST",
                    dataType:"json",
                    success:function(data) {
                      if(data){
                        $("#p").empty();  
                           ///
console.log(data); 
     
                      }else{ $("#p").empty();} 
                    },
                });
            } else {$("#p").empty();}
        });

Controller

    public function getData($a_id = null, $b_id = null, $c_id = null)
    {          
        $a = Aaa::findOrFail($a_id);
        $b = Bbb::findOrFail($b_id);
        $c = Ccc::findOrFail($c_id);  
        return response()->json(['a' => $a, 'b' => $b, 'c' => $c]);  
    }