<div id="flexbox">
  <div>1</div>
  <div>2</div>
  <div>3</div>
</div>

#flexbox {
  display: flex;
  height: 100px;
  column-gap: 20px;
  flex-wrap:wrap;
}

#flexbox > div {
  border: 1px solid green;
  background-color: lime;
  flex: 0 0 50%;
  box-sizing:border-box;
}