<div class="g-bd3 f-cb">

    <div class="g-sd31">

        <p>我是左侧</p>

    </div>

    <div class="g-sd32">

        <p>我是中间</p>

    </div>

    <div class="g-mn3">

        <div class="g-mn3c">

            <p>我是右侧自适应</p>

        </div>

    </div>

</div>

.g-bd3{

    width:980px;border:3px ridge #3f34e7;

    margin:45px auto;

}

.g-sd31,.g-sd32{

    float: left;

    width:230px;

    position: relative;

}

.g-sd31{

    width:190px;

    margin-right:10px;

}

.g-mn3{

    float: right;

    width:100%;

    margin-left: -460px;

}

.g-mn3c{

    margin-left:440px;

}

.g-bd3 p{

    height:150px;

    background-color: pink;

    padding: 5px;

}