3个DIVs，1个容器，水平居中对齐

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

.container {
  display: flex;
  justify-content: center; /* 水平居中对齐 */
}

.box {
  width: 100px;
  height: 100px;
  background-color: #ccc;
  margin: 10px;
}

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

.container {
  text-align: center; /* 水平居中对齐 */
}

.box {
  display: inline-block;
  width: 100px;
  height: 100px;
  background-color: #ccc;
  margin: 10px;
}

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

.container {
  position: relative;
}

.box {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 100px;
  height: 100px;
  background-color: #ccc;
  margin: 10px;
}