.first {
  background-color:#000;
  height:9rem;
  width:9rem;
  text-align:center;
  position:relative;
  
  color:white;
}

.second {
  background-color:#666;
  height:6rem;
  width:6rem;
  text-align:center;
  
  position:relative;
  left:50%;
  top:25%;
  transform:translate(-50%,-30%);
}

.third {
  background-color:#aaa;
  height:3rem;
  width:3rem;
  text-align:center;
  
  position:absolute;
  left:50%;
  top:50%;
  transform:translate(-50%,-50%);
  
  display:flex;
  align-items:center;
  justify-content:center;
}

let firstDiv = document.getElementsByClassName("first")[0];

firstDiv.addEventListener("click", clickEvent);

function clickEvent(event) {
	alert("clicked");
}