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>