xtabay

function replaceStars(collection) {
  var i, el, re = /^(\s*\*\s*){3}$/;
  for(i = 0; i<collection.length; i++) {
    el = collection[i];
    if( el.innerHTML.match(re)) {
      el.insertAdjacentHTML('afterend', '<hr>');
      el.parentNode.removeChild(el);
    }
  }
}

replaceStars(document.querySelectorAll('.entry-content p'));