title: How to set 3 items in row FLEXBOX
body: <p>I would like to display 3 items per row, but I want to include margin and border.
  Here's is a simple example, but I need to set valid flex property</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.box {
    display: flex;
    flex-wrap: wrap;
    width: 1200px;
  }
  
  .item {
    display: flex;
    border: 1px solid #000;
    margin: 5px;
    flex: // ???
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="box"&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
    &lt;div class="item"&gt;&lt;/div&gt;
  
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>