<div class="flex">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>


.flex {
  border: 1px solid #000;
  display: flex;
  padding: 0 4px;
}
.item {
  flex-grow: 1;
  
  box-sizing: border-box;
  margin: 8px 4px;
  padding: 64px;
  background: #faa;
}
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