<div class="block">block 1</div>
<div class="block">block 2</div>
<div class="block">block 3</div>

.block {
  display: inline-block;
  width: 100px;
  height: 100px;
  margin: 3px;
  box-shadow: 0 0 15px black;
  border-radius: 15px;
  text-align: center;
  line-height: 100px;
  cursor: pointer;
  transition: .3s;
  opacity: 0;
  visibility: hidden;
}

.block:hover,
.block:first-child,
.block:hover + .block {
  opacity: 1;
  visibility: visible;
}