flex-direction と flex-wrap をまとめて指定します。フレックスボックスについての概要は flex を参照してください。

.flexbox {
  display: flex;
  display: -ms-flexbox;
  margin: 10px;
  padding: 10px;
  border: 1px solid #999;
  background-color: #ddd;
  width: 250px;
  height: 50px;
}
.boxA, .boxB, .boxC {
  border: 1px solid #999;
  min-width: 100px;
  min-height: 20px;
  height: 22px;
  text-align: center;
}
.boxA { background-color: #fcc; }
.boxB { background-color: #cfc; }
.boxC { background-color: #ccf; }

<h5>row nowrap</h5>
<div class="flexbox" style="flex-flow:row nowrap">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>
<h5>row wrap</h5>
<div class="flexbox" style="flex-flow:row wrap">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>
<h5>row wrap-reverse</h5>
<div class="flexbox" style="flex-flow:row wrap-reverse">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>
<h5>column nowrap</h5>
<div class="flexbox" style="flex-flow:column nowrap">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>
<h5>column wrap</h5>
<div class="flexbox" style="flex-flow:column wrap">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>
<h5>column wrap-reverse</h5>
<div class="flexbox" style="flex-flow:column wrap-reverse">
 <div class="boxA">A</div>
 <div class="boxB">B</div>
 <div class="boxC">C</div>
</div>

• http://www.w3.org/TR/css-flexbox-1/#flex-flow-property
• https://developer.mozilla.org/ja/docs/Web/CSS/flex-flow
• http://caniuse.com/#feat=flexbox