HTML

<div class="parent">
  <div class="box">1</div>
  <div class="box">2</div>
  <div class="box">3</div>
</div>

CSS


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

.box {
  /*  Stretching: */
  flex: 1 1 150px; 
  margin: 5px;
  background: red;
  gap: 1rem;
}