#linkskueste {
    border: 1px solid silver;
    width: 520px;
    float:left;
    }
#mittekueste {
    border:1px solid green;
    margin-left:525px;
    margin-right:345px;
    }
#rechtskueste {
    border:1px solid blue;
    width:340px;
    float:right;
    }