<div class="container">
  <!-- 新增一个 div 把 .center 包裹起来 -->
  <div class="center-container">
    <div class="center">center</div>
  </div>
  <div class="left">left</div>
  <div class="right">left</div>
</div>

body {
  min-width: 630px;
}
.container {
  overflow: hidden;
}
.center-container {
  width: 100%;
  float: left;
}
.center-container .center {
  height: 150px;
  background-color: #94e8ff;

  margin-left: 100px; /* 新添加的属性 */
  margin-right: 200px; /* 新添加的属性 */
}
.left {
  width: 100px;
  height: 150px;
  background-color: #ffb5bf;
  float: left;
  margin-left: -100%;
}
.right {
  width: 200px;
  height: 150px;
  background-color: #8990d5;
  float: left;
  margin-left: -200px;
}