bootstrap4.css
.flex-fill {
  -ms-flex: 1 1 auto !important;
  flex: 1 1 auto !important;
}
■レイアウト:Flex d-flex:ユーティリティ:flexコンテナ全幅等幅整列 flex-fill 横幅で等幅

flex文字01
flex文字02
flex文字03
  <div class="d-flex">
    <div class="flex-fill">flex文字01</div>
    <div class="flex-fill">flex文字02</div>
    <div class="flex-fill">flex文字03</div>
  </div>