BFC3

HTML

<div class="aside"></div><div class="aside1"></div>
<div class="main"></div>

CSS

body {
        width: 300px;
        position: relative;
    }
.aside {
        width: 100px;
        height: 150px;
        float: left;
        background: blue;
     
}
  .aside1 {
        width: 100px;
        height: 150px;
        float: left;
        background: blue;
     
}   
 .main {
        height: 200px;
                background: #f00;
}