<div class="wrapper">
  <div class="item">Item One</div>
  <div class="item">Item Two</div>
  <div class="item">Item Three</div>
</div>

* {
  box-sizing: border-box;
}

.wrapper {
  background-color: rgb(79, 185, 227);
  padding: 10px;
  max-width: 400px;
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
}

.item {
  float: left;
  border-radius: 5px;
  background-color: rgb(207, 232, 220);
  padding: 1em;
  width: 33.333%;
}

@supports (display: grid) {
  .item {
    width: auto;
  }
}