function append() {
  a = document.querySelector(".classname_a");
  if (a) {
    a.style.display = "none";
    b = document.querySelector(".classname_b");
    if (b == null) {
      c = document.querySelector(".classname_c");
      str ='<div>ok</div>';
      c.insertAdjacentHTML("beforeEnd", b);
    }
  }
}
setInterval("append()", 5000);