Flexbox block-grid example

* {box-sizing: border-box;}
ul {
  list-style: none;
  padding: 0;
  margin: 0;

  display: flex;
  flex-wrap: wrap;
}

li {
  border: 1px solid red;
  flex: 0 1 33.33333%;   /* (1) */
  padding: .5em;
}