JSFiddle - React, Tailwind, and code Playground

HTML

<div class='main'>
  <div class='a'>
  45435465
  </div>
  <div class='b'>
  sfsfht
  </div>
</div>

<div class="flex_test">
        <ul>
            <li style="background: red">1</li>
            <li style="background: green">2</li>
            <li style="background: yellow">3</li>
        </ul>
        <ul>
            <li style="background: yellowgreen">4</li>
            <li style="background: greenyellow">5</li>
            <li style="background: pink">6</li>
        </ul>
        <ul>
            <li style="background: blueviolet">7</li>
            <li style="background: aliceblue">8</li>
            <li style="background: orangered">9</li>
        </ul>
    </div>
    <br>
    <br>
    <div class="float_test">
        <ul>
            <li style="background: red">1</li>
            <li style="background: green">2</li>
            <li style="background: yellow">3</li>

            <li style="background: yellowgreen">4</li>
            <li style="background: greenyellow">5</li>
            <li style="background: pink">6</li>

            <li style="background: blueviolet">7</li>
            <li style="background: aliceblue">8</li>
            <li style="background: orangered">9</li>
        </ul>
    </div>

CSS

*{
            margin: 0;
            padding: 0;
        }
        .main{
          display:flex;
          flex-direction:row;
        }
        .a{
          flex:1
        }
        .b{
          width:20px;
        }
        li{
            height: 70px;
            line-height: 70px;
            color: #000;
            background: #fff;text-align: center;
            list-style: none;
        }
        .flex_test ul {
            display: -webkit-box;
            display: -webkit-flex;
            display: -ms-flexbox;
            display:         flex;
        }
        .flex_test li {

            border-bottom: 1px solid #b5b5b5;
            border-left: 1px solid #b5b5b5;

            -webkit-box-flex: 1;
            -webkit-flex: 1 33.333333333333336%;
            -ms-flex: 1 33.333333333333336%;
            flex: 1 33.333333333333336%;
        }
        .float_test li{
            float: left;
            width: 33.333333333333336%;

        }