JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent" style="display:flex">
    <div class="div_first">1</div>
    <div class="div_second">2</div>
    <div class="div_third">3</div>
   <div class="div_next_line">
      <div class="div_first">4</div>
      <div class="div_second">5</div>
      <div class="div_third">6</div>
    </div>
    </div>

CSS

.parent {
  flex-wrap: wrap;
  width: 400px;
  border: 2px solid blue;
}
.parent > .div_next_line {
  display: flex;
}
.parent > div {
  display: inline-block;
  flex-grow: 1;
  flex-basis: calc(100% / 3);
}
.parent > div > div {
  display: inline-block;
  flex-grow: 1;
  flex-basis: calc(100% / 3);
}