<div class="grid-container">
  <div class="grid-item">1</div>
  <div class="grid-item">2</div>
  <div class="grid-item">3</div>
</div>

.grid-container {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  background-color: #ccc;
}

.grid-item {
  margin: 10px;
  padding: 10px;
  background-color: yellow;
}