https://jsfiddle.net/jonahe/2codx7ez/6/#set-as-basespace

by jonahe

HTML

<main>
  <section>
    test
  </section>
  <section>
    test
  </section>
  <section>
    test
  </section>
  <section>
    test
  </section>
  <section>
    test
  </section>
  <section>
    test
  </section>
</main>
<hr>

<h3>
  without box-sizing: border-box;
</h3>
<main>
  <article>
    test
  </article>
  <article>
    test
  </article>
  <article>
    test
  </article>
  <article>
    test
  </article>
  <article>
    test
  </article>
  <article>
    test
  </article>
</main>

CSS

main {
  display: flex;
  justify-content: space-between;
  flex-wrap: wrap;
}

section {
  width: 29%;
  border: 1px solid black;
  background: yellow;
  padding: 20px;
  box-sizing: border-box;
}



article {
  width: 29%;
  border: 1px solid black;
  background: yellow;
  padding: 20px;
}