<div class="box">
        <div>
            <p class="thumb">
              <img src="img/f1.png">
              <br>
              <img src="img/f2.png">
            </p>
        </div>
        <div>
            <p class="thumb">
              <img src="img/f3.png"> </p>
        </div>
    </div>

.thumb {height:500px;}

.thumb img  {
  display:inline-block;
  width:40%;
  margin-bottom: 10px;}

.box{white-space:nowrap;
     float:left;}

.box div{
display:inline-block;}