<div class="container">
  <div>
    first block
  </div>
  <div>
    second block
  </div>
  <div>
    third block
  </div>
  <div>
    fourth block
  </div>
  <div>
    fifth block
  </div>
  <div>
    sixth block
  </div>
</div>

body, html, .container {
  margin: 0;
  padding: 0;
}
.container {
  display: flex;
  width: 100vw;
  box-sizing: border-box;
  flex-wrap: wrap;
}
.container > div {
  flex: 0 0 auto;
  width: calc(33% - 1rem);
  margin: .5rem;
  background-color: #cecece;
  padding: 2rem 4rem;
  box-sizing: border-box;
  border-radius: .25rem;
}