Open Pop up with several buttons

I can not get several buttons to open a single popup. This is how I did it:

<div class="popup">
  <div id="myModal" class="modal">
    <div class="modal-content">
      <span class="close">&times;</span>
      <div class="row">
        <div class="col-sm-6">
          <p>Evita sanciones por el incumplimiento de la ley.
             Tu tranquilidad empieza con una tributación sana.</p>
          <img src="assets/images/flechablanca.png" alt="Diagnostico">
        </div>
        <div class="col-sm-6">
          <h2>Solicita hoy tu diagnóstico <strong>gratis</strong></h2>
          <form action="/action_page.php">
            <div class="form-group">
              <input type="email" class="form-control" id="emailsqyhh">
            </div>
            <div class="form-group">
              <input type="password" class="form-control" id="pwdsjkn">
            </div>
            <div class="form-group">
              <input type="aa" class="form-control" id="pwdfff">
            </div>
            <button type="submit" class="btn btn-primary">Submit</button>
          </form>
        </div>
      </div>
    </div>
  </div>
</div>



<script>
// Get the modal
var modal = document.getElementById('myModal');
// Get the button that opens the modal
var btn = document.getElementById("myBtn");
// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];
// When the user clicks on the button, open the modal 
btn.onclick = function() {
  modal.style.display = "block";
}
// When the user clicks on <span> (x), close the modal
span.onclick = function() {
  modal.style.display = "none";
}
// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}
</script>

This button works fine:

<button type="button" id="myBtn" class="btn btn-default"> 
<strong>SOLICITAR</strong></button>

but not the following buttons:

<button type="button" id="myBtn" class="btn btn-default">Conoce ayudamos</button>

<button type="button" id="myBtn" class="btn btn-default">SOLICITAR AHORA</button>

Thank you.