JSFiddle - React, Tailwind, and code Playground

HTML

<select class="cars">
      <option value=" ">Не выбрано</option>
    <option>Buggati</option>
    <option>Lada</option>
    <option>Aston</option>
</select>
<select class="cars">
      <option value=" ">Не выбрано</option>
        <option>Ferrari</option>
    <option>BMW</option>
    <option>Audi</option>
</select>
<select class="cars">
    <option value=" ">Не выбрано</option>
        <option>VAZ</option>
    <option>GAZ</option>
    <option>TAZ</option>
</select>
<input type="text" id="carsList">

JavaScript

var $carsList = $('#carsList'),
    carObj = {};
carObj.length = 0;
$('.cars').change(function(){
	carObj[$(this).index()] = $(this).val();
    carObj.length = Object.keys(carObj).length - 1;
    $carsList.val(Array.prototype.join.call(carObj, ', '));
})