JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">


  <div class="sections__container">

    <div class="sections__container__a">A-1</div>
    <div class="sections__container__b">A-2</div>
    <div class="sections__container__c">A-3</div>

    <div class="sections__container__a">B-1</div>
    <div class="sections__container__b">B-2</div>
    <div class="sections__container__c">B-3</div>

  </div>


</div>

CSS

* {
  box-sizing: border-box;
}

#container {
  width: 600px;
  margin: 0 auto;
  background-color: #ececec;
  padding: 10px;
  margin-bottom: 30px;
}

.sections__container {
  margin: 0 auto;
  max-width: 600px;
  display: flex;
  flex-wrap: wrap;
}

.sections__container__a {
  margin-right: 5px;
  width: calc(33.33% - 5px);
  height: 100%;
  border-radius: 4px;
  flex-grow: 1;
  background-color: #FFFFFF;
  font-size: 0.80rem;
  padding: 15px 0px 10px 0px;
  font-weight: 600;
  margin-bottom: 10px;
  text-align: center;
}

.sections__container__b {
  margin: 0 5px;
  width: calc(33.33% - 10px);
  height: 100%;
  border-radius: 4px;
  flex-grow: 1;
  background-color: #FFFFFF;
  font-size: 0.80rem;
  padding: 15px 0px 10px 0px;
  font-weight: 600;
  margin-bottom: 10px;
  text-align: center;
}

.sections__container__c {
  margin-left: 5px;
  width: calc(33.33% - 5px);
  height: 100%;
  float: left;
  border-radius: 4px;
  flex-grow: 1;
  background-color: #FFFFFF;
  font-size: 0.80rem;
  padding: 15px 0px 10px 0px;
  font-weight: 600;
  margin-bottom: 10px;
  text-align: center;
}