JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.3.min.js"></script>
Nota 1: <input class="nota" /><br/>
Nota 2: <input class="nota" /><br/>
Nota 3: <input class="nota" /><br/><br/>
Media: <input id="media" readonly/>

JavaScript

$(function(){
	$('.nota').change(function(){
      var total = 0;
      var notas = 0;
      $('.nota').each(function(){
          var nota = new Number(this.value);          
          if(nota === 0) return;          
          total = total + nota;
          notas++;
      })
      $('#media').val(total/notas);
  });
})