没写属性

.left这里的position后面没有属性，你忘记写absolute了！

.left{ width:200px; height:600px; background:#ccc; position:absolute; left:0; top:0}

.main{ height:600px; margin:0 310px 0 210px; background:#9CF}

.right{ height:600px; width:300px; position:absolute; top:0; right:0; background:#FCC;}