p标签如果是div标签的话，就是另个一个效果了，这样容易引起误会的。

<!DOCTYPE html>
<html>
<head>
   <meta charset="UTF-8">
   <title>Document</title>
   <style>
       #box1{height: 50px;background-color: red;float: left;}
       #box2{height: 100px;background-color: blue;float:right;}
       #box3{height:150px;width:300px;background-color: black;}
   </style>
</head>
<body>
   <div id="box1">盒子1</div>
   <div id="box2">盒子2</div>
   <div id="box3">盒子3</div>
   <p>fjkdlsjfkjslkjfdlskjdflkjsdklfjsljkdlljkdlsjfkjslkjfdlskjdflkjsdklfjsl</p>
   <p>fjkdlsjfkjslkjfdlskjdflkjsdklfjsljkdlljkdlsjfkjslkjfdlskjdflkjsdklfjsl</p>
</body>
</html>