FrankJS

const targetNode = document.querySelector('.cff-wrapper-ctn');
const config = { childList: true, subtree: true };

function addBorder(){
  const posts = Array.from(document.getElementsByClassName('cff-item'));

  posts.forEach(post =>{
    let count = parseInt(post.getElementsByClassName('cff-count')[0].innerHTML);
    if(count >= 3){
      post.style.border = '1px solid black';
    } 
  });
}

const observer = new MutationObserver(function(mutationList, observer){
 addBorder();
});

observer.observe(targetNode, config);

addBorder();