.left{ width:200px; height:600px; background:#ccc; 【任务1】position:absolute; left:0; top:0;}
.main{ height:600px; 【任务2】margin:0 310px 0 210px; background:#9CF;}
.right{ height:600px; width:300px; position:absolute; top:0; right:0;【任务3】; background:#FCC;}