<div class="boxes">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
</div>

.boxes {
  display: flex;
  justify-content: space-between;
}

.box {
  width: 200px;
  height: 200px;
  background-color: #CCC;
  position: relative;
  margin: 40px;
  border-radius: 5px;
}

.box:after {
  content: '';
  position: absolute;
  bottom: -10px;
  right: -10px;
  width: 40px;
  height: 40px;
  display: block;
  background-color: red;
  border-radius: 50%;
}