Flexbox is not always the solution. Float can do it here:
.card { float: left; width:calc(100%/3); /* control the width you want */ padding: 30px; border: 1px solid gray; box-sizing: border-box;}/* the below is only needed if you will use a small width */.break { clear: both;}.results { overflow: auto;}<div class="results"><div class="primary-container"><div class="card"> 1</div></div><div class="secondary-container"><div class="card">2</div><div class="card">3</div><div class="card break">4</div><div class="card">5</div><div class="card">6</div></div></div>