第一种方法:
<style>
.div1{ float: left; width: 300px; background: red; }
.div2{ float: right; width: 300px; background: blue; }
.div3{ background: yellow; }
</style>
<div class="div1></div>
<div class="div2"></div>
<div class="div3"></div>
第二种方法:
<style>
.div1{ position: relative; }
.div2{ position: absolute; left: 0; width: 300px; background: red; }
.div3{ position: absolute; right: 0; width: 300px; background: blue; }
.div4{ position: absolute; left: 300px; right: 300px; background: yellow; }
</style>
<div class="div1>
<div class="div2"></div>
<div class="div3"></div>
<div class="div4"></div>
</div>