<div class="container">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box box--selected"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
</div>

body {
  min-height: 100vh;
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}

.container {
  display: grid;
  grid-gap: 20px;
  grid-template-columns: repeat(3, min-content);
  
  &:hover {
    & > .box {
      opacity: 0.5;
      
      &--selected {
        opacity: 1;
      }
    }
  }
}

.box {
  width: 100px;
  height: 100px;
  background: tomato;
  transition: opacity 0.2s ease-in-out;
  
  &--selected {
    box-shadow: inset 0 0 0 10px rgba(0, 0, 0, 0.2);
  }
}