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

.container {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
}

.item {
  width: 200px;
  height: 200px;
  margin: 10px;
  background-color: #ccc;
}

@media (max-width: 768px) {
  .item {
    width: 100%;
  }
}